Fix: Sphinx Theme CSS file missing on doc pages

[semmel]

Without knowing how to use Read the Docs I applied some instructions from here
https://sphinx-rtd-theme.readthedocs.io/en/stable/installing.html

Still missing is pip install sphinx_rtd_theme. I don't know where to add that 🤷

I noticed theme.css is no longer used for https://mostcore.readthedocs.io/en/latest/api.html

On waybackmachine theme.css (/* sphinx_rtd_theme version 0.4.3 | MIT license */ /* Built 20190212 16:02 */) is used.

w/o the theme imo the docs look quite ugly now:

[briancavalier]

Hey @semmel 👋 Thanks for helping to fix this! With the other recent doc updates, I found that RTD had changed and now requires a config file. Without it, RTD refused to build and deploy the update docs 😞. So I just copied and pasted RTD's example starter config just to get the docs to build again, and hadn't had time to look into the theme problem.

Still missing is pip install sphinx_rtd_theme. I don't know where to add that

This page says:

Install the sphinx_rtd_theme package (or add it to your requirements.txt file):

The parenthetical seems like the key, but that page doesn't make it clear where requirements.txt should like. I searched around a bit and found this example repo, which was a huge help. It has a requirements.txt inside the docs dir, so it seems like we just need to do a couple simple things to get this PR merged:

1. Add a requirements.txt in the docs folder
2. Add the "right" dependencies to it. I don't know if just adding sphinx-rtd-theme is enough, or if we should just add everything from the example repo's requirements.txt. We could try the latter and see what happens.

[semmel]

I added the requirements.txt as in the RTD Tutorial Template.

It seems to work now: The doc pages generated from my fork look fine.

[briancavalier]

@semmel Awesome. Thank you very much for this!

[briancavalier]

Docs look great again 🎉