fix: exclude _dev from Sphinx source scanning to fix CI build

[Copilot]

• Have you updated CHANGELOG.md with relevant non-documentation file changes?
• Have you updated the documentation for this change?

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The CI job "Test on docs directory" was failing because Sphinx scanned docs/_dev/node_modules/ (populated by pa11y-install), generating 1,234 warnings from npm package README files, which caused the build to fail under --fail-on-warning.

Fix: Add _dev to exclude_patterns in docs/conf.py:

exclude_patterns = [
    "doc-cheat-sheet*",
    ".venv*",
    "_dev",        # prevents scanning node_modules READMEs and other tooling artifacts
]

This mirrors the existing .venv* exclusion — both directories are build tooling, not documentation source.

[jahn-junior]

I'm working on a fix in canonical/sphinx-stack#610, which will be merged by the end of this pulse. This particular change is pretty low-impact, but in general, let's be careful not to introduce much of a delta between the sphinx-stack and the docs.

[jahn-junior]

Closing this in favor of #23 so we also get the fix for post-pa11y checks (e.g., pa11y -> spelling). It's a subtle gap in our tests because pa11y always runs last.