Docs: Use "Sentence case" for titles

[benjaoming]

@ericholscher This is supposed to be a 100% neutral change. No discussions of titles intended. Just want to get sentence case introduced and move on to creating new sub-levels in the next PR...

• Go through all titles
• Go through all "How to do x" titles
• Navigation labels for all "How to do x" titles...
• Go through all references like :doc: and :ref: that has a "Title Case" reference

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📚 Documentation previews 📚

• User's documentation (docs): https://docs--10055.org.readthedocs.build/en/10055/

• Developer's documentation (dev): https://dev--10055.org.readthedocs.build/en/10055/

[benjaoming]

I'm investigating why I'm getting a lot of undefined label warnings.

The build of this PR is broken for good reasons.

But for instance this build (where we merged all the Diátaxis changes) did not fail, although it contained the same broken references.

Example:

/home/docs/checkouts/readthedocs.org/user_builds/docs/checkouts/10055/docs/user/analytics.rst:22: WARNING: undefined label: 'server-side-search/index:search analytics'

server-side-search/index:search analytics clearly doesn't exist. But it wasn't refactored in this PR.

This means that this page currently contains a broken anchor link:

https://docs.readthedocs.io/en/stable/analytics.html

[benjaoming]

A quick pattern for ack to find all occurrences of title case :doc: and :ref:

ag -I ":doc:\`\\p{Lu}\pL+\s\\p{Lu}"
ag -I ":ref:\`\\p{Lu}\pL+\s\\p{Lu}"

[ericholscher]

Simple enough 👍