docs: integration to single page

* Improves PEP8 and PEP257 of all files.

* Integrates documentation on a single page.

* Adds tests for strict Sphinx documentation build and PEP257.

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
menu: additional keyword arguments store

* Stores additional keyword arguments as `MenuItem` attributes.

Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Signed-off-by: Eirini Psallida <eirini.psallida@cern.ch>