make readthedocs documentation awesome

[drbenvincent]

#53 added sphinx documentation and got it working reliably on readthedocs. This issue focuses on improving the docs and making them look nicer!

Focussing mostly on the index page:

• Better page title
• Double check best location for images (and other static resources) for the docs
• Add logo when CausalPy logo #27 is complete
• Add some more compelling content, taking inspiration from the GitHub README. This should include images.
  • Since the move to separate notebook examples, we not longer have auto image export. So it would be good to create a script to generate images for the documentation.
• Add a quickstart
• Rather than itemise individual examples in the sidebar, switch to having one page of example. So there will be one Examples entry in the sidebar which leads to a single page which will expand with examples over time. Also see SC: in-depth notebook #35, ITS: in-depth notebook #36, DiD: in-depth notebook #37, RDD: in-depth notebook #38, and Add examples for 'classic' causal inference datasets #44.
  • Fix any links to old notebooks in the README
• Remove module index and search page
• Add info relating to PyMC Labs supporting the package
• opt out of remote execution of notebooks on readthedocs
• Can we improve/simplify the importing of data with pathlib. Current method looks clumsy

[drbenvincent]

After a lot of faffing about with remote build of readthedocs, we have the new structure (with separated small notebooks) up and running https://causalpy.readthedocs.io

[drbenvincent]

Closing this. Only substantive remaining issue is the logo, but that's dealt with in #27