docs(python): Improved structure for DataFrame and LazyFrame API docs, misc design improvements

[alexander-beedie]

Design Update

Light mode:

• Header navbar (and footer) background colour now matches the ice-blue titlebar of the homepage.

Dark mode:

• Polars logo (top-left) no longer displays as black on black.
• Titlebar colour now more visible.
• Less stark border contrast.

Both modes:

• Left-hand sidebar nav has a subtle background tint to better distinguish from the main body.

Improved navigation/structure

• DataFrame and LazyFrame nav structure now updated similarly to the previous PR for Series and Expr.
• Each of the major classes now has a more useful root page; rather than a bare list of toctree links it now contains all the methods and a neat summary table under the main class init method; handy one-stop-shop for search/lookup.
• Side navigation breaks-out the operations by the same functional categories that were present in the previous .rst for each of the classes.

Misc

• Hid the rightmost toctree sidebar as it doesn't show anything on most pages, and the additional space is more useful.
• Updated LazyFrame class docstring so the associated page has a useful lead-in.
• Slightly improved the _minify_classpaths regex in Sphinx conf.py.

[ritchie46]

Good improvements! Could you store/take the images in/from https://github.com/pola-rs/polars-static? That way we don't have to store binary files in this repo.

[alexander-beedie]

Good improvements! Could you store/take the images in/from https://github.com/pola-rs/polars-static? That way we don't have to store binary files in this repo.

Done; once they're up I'll tweak this one accordingly 👍
(Update - ready to roll).