Automate creating releases from Changelog and add Changelog to docs

[edwardchalstrey1]

Issues closed by this PR

• Closes [Documentation]: Improve & automate release notes to not rely on GitHub #712

Description of the changes in this PR

This PR:

• Updates the install page with clearer instructions and links to the releases
• Shows the ChangeLog in the docs via a symlink on the install page and myst-parser, so release history is part of the documentation site rather than only available on GitHub.
• Automate GitHub release creation with a new .github/workflows/release.yml workflow.
  • When a vX.Y.Z tag is pushed, the workflow extracts the matching section from ChangeLog and creates the GitHub release with those notes - no need to copy and paste
  • Updates the developer release doc (doc/developer.contributing.rst) to reflect that step 6 (GitHub release creation) is now automated, and to clarify that ChangeLog is the single source of truth for release notes.
• The extraction script which gets the info from the Changelog for the docs (used by the GH workflow) is added in build_support/releases with tests. This ensures the Changelog is always formatted correctly (also explained in contributor docs)
• Adds myst-parser to the documentation dependencies (pyproject.toml and doc/conf.py) to support Markdown source files in Sphinx.

How to review this PR

• Open PR preview of documentation:
  • Look at the install page, which links to the Changelog: https://gambitproject--954.org.readthedocs.build/en/954/install.html
  • Look at the "Making a new release" section of contributor docs page: https://gambitproject--954.org.readthedocs.build/en/954/developer.contributing.html#releases-maintenance-branches
• GitHub Actions workflow: Review .github/workflows/release.yml and build_support/releases/extract_changelog.py