Extract "testing" section from readme + Add section "development environment"

[vjik]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌
Fixed issues	comma-separated list of tickets # fixed by the PR, if any

[what-the-diff]

PR Summary

• Introduction of Internal Documentation
  A new file called docs/internals.md has been added. This document will help to understand more about the project's inner workings. It covers various aspects such as setting up the development environment, conducting unit and mutation testing, using static analysis, following the code style, and managing dependencies.

• Simplified Installation Instruction
  The command needed to incorporate the yiisoft/cache-memcached package has now been simplified in the README.md. The --prefer-dist flag has been removed to make the command more straightforward.

• Enhanced Support Information
  The "Support the project" section in README.md has been updated with added links to Open Collective for sponsorship, making it easier for those who wish to financially support the project.

• Extended Follow-Up Options
  People interested in following updates on the project now have a broader range of platforms to choose from. Links to the project's official website, Twitter, Telegram, Facebook, and Slack have been added to the "Follow Updates" section of README.md.

• Updated License Information
  The "License" section has been removed from README.md for simplification. However, it's been specified that the project is released under the BSD License, and a direct link to the LICENSE.md file is provided.

• Clarification on Project Maintenance
  The README file has been updated to clearly mark Yii Software as the maintainer of the project in the "Maintained by" section.