build: add Documentation linting step to CI

[pomegranited]

Description

#32741 removed the documentation build step because it was taking > 40 minutes to run.

This PR adds a "documentation lint" step to the CI which uses sphinx-lint. This doesn't build the whole documentation tree, but should let PR creators know if they've made any code changes that break documentation.

This change affects Developers only.

Supporting information

See #35766 (comment) for more context.

Testing instructions

1. Ensure the "Documentation linter" step added to the CI build is passing.

To verify that this added step helps us:

1. Revert the change made by fix: adds content.search to docs apps #35766
2. In a python3.11 virtualenv, run:

   pip install setuptools
   pip install -r requirements/edx/doc.txt
   

3. make docs-lint should fail with the error reported in the referenced PR.

Deadline

None

[openedx-webhooks]

Thanks for the pull request, @pomegranited!

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[pomegranited]

Closing this PR as my CC time for this year has run out, and Sumac docs are building again :)