Add issue and PR templates.

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve SLEEF
+title: ''
+labels: ''
+---
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**Command lines and logs**
+ - CMake configuration AND build command
+ - CMake configuration logs
+ - CMake build logs (only the relevant part)
+
+**To Reproduce**
+Always try to provide a minimal reproducer, wherever possible.
+ - Link to a [Compiler Explorer](https://godbolt.org/) reproducer, if applicable.
+ - Provide source code for reproducer along with command used to build and run reproducer.
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Screenshots**
+Consider attaching screenshots to help explain your problem.
+
+**Environment**
+ - Is it a native build or cross-compilation?
+ - Host/Target Architecture [e.g. x86_64 if native build, x86_64/aarch64 if cross build, ..]
+ - OS with version [e.g. Linux/Ubuntu-22.04, Windows, macOS, iOS, ...]
+ - CMake version [e.g. 3.18, ...]
+ - Makefile Generator [e.g. GNU Make, Ninja, ...]
+ - Compiler with version [e.g. gcc-14, clang-18, apple-clang-15.2, msvc-15.0, ...]
+ - If CUDA, please provide the version of the CUDA ToolKit.
+
+**Additional context**
+Add any other context about the problem here.
+

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links:
+  - name: 🗣 Ask a Question, Discuss
+    url: https://github.com/shibatch/sleef/discussions
+    about: Get help using SLEEF

.github/ISSUE_TEMPLATE/docs_request.md

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+---
+name: Request Documentation
+about: Request a change or report an issue in documentation
+title: ''
+labels: ["doc"]
+---
+
+**Where?**
+Where in the documentation did you notice an issue, e.g. `README.md`, `docs/*.md`, `sleef.org/*`, ...
+
+**What?**
+What issue do you want to report? What change do you want to make?
+
+**How?**
+A clear and concise description of what you want to change/update.
+
+**Additional context**
+Add any other context or screenshots about the issue here.

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest a new feature or an idea for SLEEF
+title: ''
+labels: ''
+---
+
+**Why a new feature? Is your feature request related to a problem? Please describe it.**
+A clear and concise description of what the problem is, e.g.,
+- I'm always frustrated when [this happens]
+- [This program] that uses SLEEF fails [in some way], because it is failing to provide [...]
+
+**What feature would you like?**
+A clear and concise description of what feature you want to introduce.
+Is it related to: algorithms, architecture specific code, build system, performance, accuracy, ...
+
+**How would you like this feature to be implemented?**
+A clear and concise description of how you want the feature implemented.
+
+**How portable is the feature across architectures and platforms?**
+A clear and concise description of any potential portability issues.
+
+**Describe alternative approaches you have considered**
+A clear and concise description of any alternative approaches you have considered.
+
+**Additional context**
+Add any other context or screenshots about the feature request here.

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- Thank you for contributing! -->
+
+# Checklist
+
+- [ ] I have read the [contributing guidelines](https://github.com/shibatch/sleef/blob/HEAD/CONTRIBUTING.md).
+- [ ] I have considered portability of my change across platforms and architectures.
+- [ ] I have self-reviewed my code.
+- [ ] I have commented my code where necessary.
+- [ ] I have updated the documentation accordingly.
+- [ ] I have added tests that prove my fix is effective or that my feature works.
+
+# What is the purpose of this pull request?
+
+<!-- Keep only the lines that apply. -->
+
+* Fix a bug
+* Improve code quality or performance
+* Add support for a new vector extension
+* Add a workflow to GitHub Actions or add/modify a job in an existing workflow
+* Documentation update
+* Other, please explain:
+
+# What changes did you make?
+
+<!-- Give an overview of the change. -->
+
+This PR consists in...
+
+# Does this PR relate to any existing issue?
+
+Relates to/Fixes # (issue)
+
+# Is there anything you would like reviewers to focus on?
+
+Please on focus on...

CONTRIBUTING.md

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+# Contributing
+
+Please be sure to read the contribution guidelines before making or requesting a change.
+
+## Design Principles
+
+Before anything please consider what are the main design principles of SLEEF:
+
+1. Portability across platforms and architectures
+2. Compliance with C99 standard
+3. Best performance of vector implementation of branch-free and lookup-free algorithms
+
+## Filing Issues or Requesting a Change
+
+Before filing a report, please be sure to read the guidelines for what you are reporting:
+
+* [Report Bugs](https://sleef.org/6-contribute#report-bugs)
+* [Request Feature](https://sleef.org/6-contribute#request-feature)
+* [Request a Change in Documentation](https://sleef.org/6-contribute#request-documentation)
+
+## Contributing Code or Documentation
+
+Before submitting code or documentation to SLEEF, please read over the [Pull Request Guidelines](https://sleef.org/6-contribute#pull-requests).
+
+## Full Documentation
+
+Our full contribution guidelines can be found at: <https://sleef.org/6-contribute>

docs/6-contribute/README.md

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+---
+layout: default
+title: Contribute
+nav_order: 7
+permalink: /6-contribute/
+---
+
+# Contribute to SLEEF
+{:toc}
+
+## Discussions
+
+If you have a question that does not necessarily require a change to SLEEF,
+such as asking how something works or how to contribute, please [open a
+discussion](https://github.com/shibatch/sleef/discussions/new/choose).
+
+## Templates
+
+By using the provided issue or pull-request template you make sure we have all
+the information we need to triage, understand and solve your issue as quickly
+and efficiently as we can. Please help us by providing as much information as
+you can by filing the provided forms.
+
+## Report Bug
+
+If you would like to report a bug in SLEEF, please [create a new
+issue](https://github.com/shibatch/sleef/issues/new) and select [Report
+Bugs](https://github.com/shibatch/sleef/issues/new?template=bug_report.md).
+You can also suggest a fix by [submitting a pull
+request](https://github.com/shibatch/sleef/pulls) on GitHub.
+
+## Request Feature
+
+If you would like to request a feature or a change to SLEEF, please [create a
+new issue](https://github.com/shibatch/sleef/issues/new) and select [Request
+Features](https://github.com/shibatch/sleef/issues/new?template=feature_request.md).
+
+## Request Documentation
+
+If you would like to report an issue or request a change to the documentation,
+please [create a new issue](https://github.com/shibatch/sleef/issues/new) and
+select [Request
+Docs](https://github.com/shibatch/sleef/issues/new?template=docs_request.md).
+
+## Pull Requests
+
+If you would like to suggest a fix or any change to SLEEF, please [submit a
+pull request](https://github.com/shibatch/sleef/pulls) on GitHub.