[sphinx docs] Formatting improvements

[johny-b]

This should be done after #623 .

Make the docs as pretty as possible (without influencing the merit of the docs).

This includes:

• better formatting
• adding "small" missing parts (e.g. some functions might be missing params or return sections, or more likely have them written in a non-formal way)
• moving some parts of the docstrings (e.g. from Golem docstring) to the main docs file
• internal restructuring (e.g. splitting the docs into more than one file)

DETAILS (as of aff21b1):

1. Duplicated external links in menu
   -> done in ee649c0
2. Golem.execute_tasks is rendered as async - why? Probably a bug in sphinx, removing yield helped. Maybe report a bug? Or maybe there's a way to fix this?
   -> I couldn't fix this, I guess this is a bug in sphinx. I created an issue: autodoc: Asynchronous Generators are marked as non-async functions sphinx-doc/sphinx#9639
3. Find all functions in docs without specified params & fix them
4. Read carefully once again