PR Summary

Skip the checks on Azure Pipelines when the changes are all underdoc/ and/orgalleries/. We already did this for Github Actions at#25261 and Appveyor at#25383, so this completes the set.

Relevant Azure docs:

• Including or excluding paths
• More details on path/wildcard matching

Also added a small note to the dev guide about this skip being automatic. Since this PR has changes both inside and outside the "exclude" directories, the tests should still run. This seems worth checking as I couldn't see anything in the Azure docs that spells out what happens in this situation (but I would hope that it's consistent with GHA and Appveyor).

PR Checklist

Documentation and Tests

• Has pytest style unit tests (andpytest passes)
• Documentation is sphinx and numpydoc compliant (the docs shouldbuild without error).
• New plotting related features are documented with examples.

Release Notes

• New features are marked with a.. versionadded:: directive in the docstring and documented indoc/users/next_whats_new/
• API changes are marked with a.. versionchanged:: directive in the docstring and documented indoc/api/next_api_changes/
• Release notes conform with instructions innext_whats_new/README.rst ornext_api_changes/README.rst