Move plugin directory into documentation !stable-docs

[ColumbusLabs]

Summary

• move the maintained sqlite-utils plugin list into a native Sphinx plugin-directory page
• link the directory from the plugin guide and main documentation navigation
• replace the deprecated sqlite-migrate listing with the built-in migrations documentation
• add the established stable-docs workflow so marked documentation updates can reach /stable/ without a package release
• make first-run branch creation and later documentation deletions or renames safe

Closes #601.

Verification

• uv run pytest — 1,369 passed, 19 skipped
• uv run mypy sqlite_utils tests
• uv run flake8
• uv run ty check sqlite_utils
• uv run black . --check
• uv run cog --check --diff README.md docs/*.rst
• uv run --group docs codespell docs/*.rst --ignore-words docs/codespell-ignore-words.txt
• Sphinx HTML build; new page and both navigation paths inspected
• every external link added by the directory returns HTTP 200
• local bare-remote workflow simulation covers first run with no stable branch, same-commit !stable-docs propagation, a documentation deletion, and a 100% rename
• YAML syntax and whitespace checks

Sphinx -W reaches a single warning in docs/changelog.rst:14; the identical warning reproduces on untouched origin/main. The normal HTML build succeeds with the new page.

Merge note

Please preserve !stable-docs in the pushed head commit message (the PR title includes it for squash merges) so this page is copied to the stable branch immediately.

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📚 Documentation preview 📚: https://sqlite-utils--813.org.readthedocs.build/en/813/