Docs: Add some messages flagging the upcoming requirement of a .readthedocs.yaml

[benjaoming]

These are just helpful pointers, documentation should be improved afterwards

Fixes #10372

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📚 Documentation previews 📚

• User's documentation (docs): https://docs--10389.org.readthedocs.build/en/10389/

• Developer's documentation (dev): https://dev--10389.org.readthedocs.build/en/10389/

[ericholscher]

These changes look good other than the tutorial, I think we want to prioritize an update there, not a warning that is a "dead end" for the user.

[benjaoming]

I think this section rename works well for the tutorial. It speaks more directly to the actionable item.

[ericholscher]

Looks good with a small edit.