Unify documentation GitHub/ReadTheDocs

[ClaudiaComito]

Related
#1186

Feature functionality
At this time our infos on the GitHub repo, esp. onboarding and contributing instructions, have diverged from the ReadTheDocs docs

We need to unify docs location and cross-references so the info is consistent and updates unambiguous.

Existing markdown instructions on this repo should be ported to .rst and moved to doc/. Updates to our README.md should be automatically picked up in docs/introduction.rst. And so on.

This is a mid-term goal. I think it's OK if it doesn't get solved immediately, as long as the solution no longer involves manual porting from GitHub to ReadTheDocs and viceversa.

[github-actions]

This issue is stale because it has been open for 60 days with no activity.

[ClaudiaComito]

This issue is still open.

We now have new jupiter notebooks #1406 in the tutorials directory. What still needs to be done here:

• implement sphinx tutorials as jupyter notebooks (all done but dpnn)
• have sphinx source files point to notebooks https://docs.readthedocs.io/en/stable/guides/jupyter.html (thanks @JuanPedroGHM )
• README, getting started, contributing etc: there should be one source file for all of these (unclear what's the best way to do it)

[mtar]

MyST looks quite interesting
https://myst-parser.readthedocs.io/en/latest/index.html#