Tracking DF 40 release

[Michael-J-Ward]

Remaining work being done for #771

• fix find_window_function failures on tpch and docs generation
• approx_percentile_cont can't accept num_centroids  #777
• test_regr_funcs does not test datafusion-python functions #778
• ensure examples still run
• fix or create issues for renaming todo comments
• Bugfix: Calling count with None arguments #768
• Add in user example that compares a two different approaches to UDFs #770
• Add missing exports for wrapper modules #782
• Doc/cross reference #791
• Cannot convert Expr to InList #781

[timsaucer]

I think we should add in this PR to release 40. #782 It adds in the missing exports. I don't think any of these are strictly necessary from my testing but it's possible some other users are importing these classes.

This one is a nice to have: #791 where it updates the documentation site to include cross references.