Improve Python documentation

[felixhekhorn]

less serious than #384 , but more serious than #377 I'd say

1. most importantly: Python is missing a huge junk of information, in particular there is no information on boc - why? I wonder if there is a problem with rtd? when did it last update?
2. Improve Rust documentation #384 also applies to Python of course (and any other API for that matter)

[Radonirinaunimi]

1. most importantly: Python is missing a huge junk of information, in particular there is [no information on `boc`](https://pineappl.readthedocs.io/en/latest/search.html?q=boc&check_keywords=yes&area=default) - why? I wonder if there is a problem with rtd? when did it last update?

This might indeed hint to a problem with RTD. I will check it when updating the documentation.

[felixhekhorn]

I don't know what is happening, but the version on rtd is 1.3.3 = current, so it is up-to-date. Does one have to explicitly tell what is exposed here https://pineappl.readthedocs.io/en/latest/modules/pineappl.html ? as for the content: I think following the current practice of pointing to the Rust equivalent ("PyO3 wrapper to ...") is a safe way to keep the two in sync (and bubbles #384 immediately to here)