documentation: deploy docs website to readthedocs

[superlopuh]

This PR sets up readthedocs hosting for our mkdocs docs, meaning we get the best of both worlds:

• docs for each version of xDSL from now on available in a drop-down menu on a platform we don't need to manage (and maybe even for each PR but I haven't tried that yet)
• we control how the docs are generated and structured in MKDocs's API, which is more flexible and modern than the readthedocs default Sphinx

Check it out here:

https://xdsl.readthedocs.io/en/latest/

We definitely want to clean it up and make it nicer before making public announcements but it seems like a big step forwards.

[superlopuh]

One more thing is that we can still use gh pages for something else if we decide to host on readthedocs, not sure that the current jupyterlite page is what we want, but it can stay up a little bit longer while we decide.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.23%. Comparing base (e209b0c) to head (c5572d6).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4043   +/-   ##
=======================================
  Coverage   90.23%   90.23%           
=======================================
  Files         458      458           
  Lines       58409    58409           
  Branches     5691     5691           
=======================================
  Hits        52707    52707           
  Misses       4320     4320           
  Partials     1382     1382           

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[compor]

Navigation could use some work, but that's a great start!

[compor]

I had to look up https://docs.readthedocs.com/platform/stable/reference/environment-variables.html#envvar-READTHEDOCS_VIRTUALENV_PATH. Not sure if it's worth adding it as a comment in similar fashion to the existing comment above.