build: Restore documentation and test files in sdist

[adrienverge]

This commit fixes a problem reported by Nicholas Bollweg at #725. Recent commit a3e1325 "CI: Publish PyPI releases using GitHub Actions workflows" and automatic publication of version 1.36.1 revealed a problem that had probably been there for some time: most documentation and test files weren't included in the source distribution when running python -m build (unless an yamllint.egg-info from a previous run listed them).

I see two solutions:

1. Add plugin setuptools-scm in pyproject.toml:

   requires = ["setuptools >= 61", "setuptools-scm >= 8"]

   This would add all files tracked by Git in the sdist, including a few that aren't really needed: .flake8, .github, .readthedocs.yaml…
2. Declare extra files to embed in MANIFEST.in. This is what this commit does.

I checked that:

• All files packages before 1.36.0 are correctly included now:

  tar -tvf dist/yamllint-1.36.0.tar.gz | cut -b65- \
    > /tmp/sdist-files-before
  rm -rf yamllint.egg-info && python -m build && \
    tar -tvf dist/yamllint-1.36.1.tar.gz | cut -b65- \
    > /tmp/sdist-files-after
  git diff --no-index /tmp/sdist-files-before /tmp/sdist-files-after

• These extra files are still not installed by pip install (they are not needed):

  pip install yamllint==1.36.0 && \
    tree ~/.local/lib/python3.13/site-packages/yamllint \
    > /tmp/pip-install-before
  pip install dist/yamllint-1.36.1.tar.gz && \
    tree ~/.local/lib/python3.13/site-packages/yamllint \
    > /tmp/pip-install-after
  git diff --no-index /tmp/pip-install-before /tmp/pip-install-after

[coveralls]

coverage: 99.825%. remained the same
when pulling 6147922 on dist/fix-files-not-included-in-sdist
into 27d05bd on master.