Document the existing local newsfragment check

[anshurajbisoyi98-ctrl]

Fixes #3084.

Document the existing command for checking newsfragment syntax and references locally:

make -C docs dummy SPHINXOPTS="-E -W --keep-going"

The final diff is 11 added lines in newsfragments/README.rst. It reuses the existing Makefile and Sphinx configuration, with no custom builder mode, Python changes, or new test infrastructure. The dummy builder checks the assembled changelog and documentation without generating HTML; -E rereads edited fragments and -W makes warnings fail the build.

Validation by Codex on macOS / Python 3.13.12:

• The documented command passes with valid fragments.
• An intentionally undefined reference makes it fail.
• Hash comparisons confirm existing fragments and release history remain unchanged; the Git index tree is preserved.
• Applicable pre-commit hooks and git diff --check pass.

This replaces the earlier implementation in response to the concern about maintenance cost. Fresh CI on a2b00b7 is pending.

AI assistance: I used Codex to simplify the change, verify the commands, and update this description.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (582223a) to head (a2b00b7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3530   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             128          128           
  Lines           19474        19474           
  Branches         1323         1323           
===============================================
  Hits            19474        19474           

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[anshurajbisoyi98-ctrl]

Addressed the four uncovered lines reported by Codecov in a8400c3.

I extracted the documentation-config lookup and added cases for checkout tests, installed-package tests running from empty/, and missing documentation sources. I also exercised the subprocess guard to confirm it raises a failure if the check tries to run Towncrier or Git.

Local verification with Codex on Python 3.13.12:

• Tool tests: 22 passed, 3 skipped.
• Without optional Sphinx dependencies: the module correctly skips during collection.
• Combined coverage for test_check_newsfragments.py: 100% lines and branches (70 statements, 12 branches; no missing lines or partial branches).
• Applicable pre-commit hooks, Mypy, make -C docs check-newsfragments, and git diff --check passed.

The new commit is pushed; the refreshed Codecov report and CI results are pending.

[A5rocks]

Sorry this seems significantly too much code for very marginal value, to me.

[anshurajbisoyi98-ctrl]

Thanks, I agree the custom mode and test infrastructure were too much for this convenience. I simplified the PR in a2b00b7 to an 11-line README addition documenting the existing Sphinx dummy-builder command. There are now no Python or Makefile changes in the final diff.

I verified that the command succeeds on valid fragments, rejects an undefined reference, and preserves existing fragments, release history and the Git index. Applicable lint checks pass. I also updated the title and description to match this smaller scope.

Simplification and verification performed with Codex.

[A5rocks]

Thanks for this PR! I think it's true that we need to improve this section of the contributor docs, but I think it would be rude to new contributors to give them LLM output as contribution docs.

[A5rocks]

I don't think we need to provide -W for this? And the description for -E doesn't help me understand why that's there.