Various improvements to documentation infrastructure

[OriolAbril]

Multiple changes related to documentation infrastructure.
Can add some more depending on preferences.

• Add conda forge install instructions

• add viewcode and copybutton extensions. They add the [source] link to docs and a copybutton to
  copy whole cells. I will double check, but using the copy button should also copy the snippets
  in docstrings like the one in https://causalpy.readthedocs.io/en/latest/api_skl_experiments.html
  without the >>> parts.

• Add the rtd-preview workflow that adds a link to the PR documentation preview as text at the
  bottom of the PR description. From what I see, PR doc preview aren't active yet, this can
  be done from https://readthedocs.org/dashboard/causalpy/edit/, scroll down and check
  the

  Build pull requests for this project:

  • More information in our docs.

• remove sphinx-design from [docs] external dependencies as it was not used

• updates to rtd config to make it more future proof

Related to #236 (not completely sure about the scope of the issue so not sure if it should be closed already or not yet), closes #86

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.98%. Comparing base (e092650) to head (7a23593).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #354   +/-   ##
=======================================
  Coverage   79.98%   79.98%           
=======================================
  Files          21       21           
  Lines        1634     1634           
=======================================
  Hits         1307     1307           
  Misses        327      327           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[drbenvincent]

Nice. So far with a local build of the docs... Copy button works great. Though it doesn't get rid of the >>> and ... parts from the examples in docstrings in the API documentation. It would be cool if that is possible.

I've also enabled that function on readthedocs. Though I guess we can't assess it works until this is merged.

Did you have any other things you wanted to add, or shall I approve this?

[OriolAbril]

Still needs some work. I'll fix the copybutton and preview action. Do you want me to also add commonly referred libraries to intersphinx and start using more cross-references? Is there a page that come to mind that would be a candidate to serve as kind of example/template on how to use it?

[drbenvincent]

In terms of the cross-references, I think this is most useful in the API pages. An example would be the causalpy.pymc_experiments page

For example the dataframe type is highlighted but not clickable to pandas. At the moment the pymc model is not type hinted, but that would be another cross-library reference.

[OriolAbril]

Should be ready to merge now

[drbenvincent]

Thanks @OriolAbril!