Add experiment design notation docs page to new Knowledge Base section of the docs

[drbenvincent]

• In this discussion https://github.com/pymc-labs/CausalPy/discussions/262 I floated the idea of adding some didactic information to the docs. This PR represents my first attempt to do that.
• It adds a new section on the left navbar, called Knowledge Base
• And it adds a new page which runs through the tabular experiment design notation established in Shadish Cook & Campbell (2002) and Reichardt (2009).
• Happy to have some feedback

[codecov[bot]]

Codecov Report

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

Project coverage is 77.10%. Comparing base (a64fc0a) to head (0db64fc).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #312 +/- ## ======================================= Coverage 77.10% 77.10% ======================================= Files 21 21 Lines 1380 1380 ======================================= Hits 1064 1064 Misses 316 316 ```

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[drbenvincent]

Failing doctest seems to be some broken import in arviz, nothing internal to causalpy.

[drbenvincent]

I left some comments. Let mw know what you think. I think it would bring a lot of clarity to use the potential outcomes language (just a suggestion)

It would also be nice to indicate briefly after eash method how this could be done in CausalPy. I think it would be a great entrypoint for the library.

Thanks for the review. It's just my edit to the ANCOVA/covariate(s) which might need to be looked at I think.

For the moment I'm trying to steer clear of mentioning the potential outcomes framework, or talk about DAGS and backdoors etc. My rough goal here is to create a series of relatively self-contained knowledge base pages which are relatively focussed. So this docs page is intended to focus on the experiment design side of things, but there will be another docs page focussing on DAGS for the different quasi-experimental designs, and maybe others on the potential outcomes framework or g-computation. Similarly, I'm trying to keep some separation between the theory (in the knowledge base) and practice (in the example notebooks). It might not always be like that, but at the moment that seems like the right structure to tackle things in relatively bite-sized chunks

[juanitorduz]

Got it! Thanks for providing context!

[juanitorduz]

BTW: The test is failing because of https://github.com/pymc-labs/pymc-marketing/pull/608. An arviz release will fix it.

[drbenvincent]

Remote tests still failing despite there being an arviz release (https://github.com/arviz-devs/arviz/releases/tag/v0.18.0) 12 hours ago