Make sections for subcommands of `github` linkable in readthedocs

[PawelLipski]

As for now, the subcommands like create-pr can't be linked directly... just the entire github command can be linked to by https://git-machete.readthedocs.io/en/stable/#github.

Consider the same for other commands/help topics, like go, hooks, list, show.

[PawelLipski]

Getting this sorted out in HTML is easy, just enough to replace ---- under subsection titles with ~~~~ (see https://docutils.sourceforge.io/docs/user/rst/quickstart.html#sections).

The main part of this task is to rework docs/generate_py_docs.py so that it properly renders the nested subsections into text format.