Improve docstrings in GP module

[daniel-saunders-phil]

This is a new PR that finishes the work from #6609. I ran into difficulties to resolve git issues and Oriol and I decided opening a new PR would be the best bet.

A summary of doc changes:

• implemented style changes
• properly named object types like string -> str,
• fixed references to other pymc objects like Zero -> ~pymc.gp.mean.Zero
• clarified default arguments where they weren't listed
• added description for some missing arguments like the model argument in the predict methods

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📚 Documentation preview 📚: https://pymc--6652.org.readthedocs.build/en/6652/