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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

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

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[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