Paper PR workflow: sync/resize images to PySR_Docs

[MilesCranmerBot]

Adds a label-gated (paper-image-sync) pull_request_target workflow to handle paper PR images safely.

• Detects newly added images under docs/src/public/images/ in the PR
• Downloads + resizes/compresses (max width 800px, quality 80)
• Converts images to JPEG always (alpha flattened to white)
• Uploads processed images to MilesCranmer/PySR_Docs (images/) and opens a docs PR
• Comments back with raw.githubusercontent.com/.../images/<file>.jpg URLs to paste into docs/papers.yml
• If the PR branch is in-repo (not a fork), can also remove the images from the PR branch and update docs/papers.yml

Requires adding secret PYSR_DOCS_TOKEN to MilesCranmer/PySR with write access to MilesCranmer/PySR_Docs.

[MilesCranmerBot]

Added a simple-but-robust validator + CI job:

• new scripts/validate_papers_yml.py checks docs/papers.yml parses + basic schema, and validates image: is either http(s) or an existing basename under docs/src/public/images/.
• CI job Validate docs/papers.yml runs on PRs.

This should catch the common formatting issues you’d otherwise check manually.

[MilesCranmerBot]

Update: adjusted the validator so it does not accept arbitrary image URLs. If image: is a URL, it must be the stable raw.githubusercontent.com/MilesCranmer/PySR_Docs/(master|refs/heads/master)/images/... form (and it still rejects the temporary paper-images/pr-... URLs). Otherwise, authors should upload the image in the PR and set image: to the local filename under docs/src/public/images/.