Improve directories layout: version subdirs and stable symlink

[Copilot]

Summary

Improves the GitHub Pages published directory layout:

• Per-version subdirectories: Format files are now stored under _site/<major.minor>/ (e.g. _site/3.14/) instead of the flat _site/3/ directory. The major.minor is obtained via python -c 'from tools.extensions.patchlevel import get_version_info; print(get_version_info()[0])' in the Doc dir, which always returns just X.Y (unlike dist_version which can return X.Y.Z for final releases).

• Stable symlink: In the scheduled deploy job, a relative symlink 3 -> <stable> (e.g. 3 -> 3.14) is created, where the stable version is the first entry with status == "bugfix" from the release-cycle JSON.

Changes

build.yaml

• Added major_minor output to the prepare job alongside existing dist_version
• Updated publish job to place files in _site/${{ needs.prepare.outputs.major_minor }}/

schedule.yaml

• Updated deploy job "Copy archives" step to extract major_minor from each artifact filename using sed and copy to the appropriate _site/<major_minor>/ subdirectory
• Added "Symlink 3 to stable version" step that creates _site/3 -> <bugfix_version> with error handling if no bugfix version is found