Add logo to readthedocs and add favicon

[jpreszler]

This resolves issue #237 .

I'm using the same image for the favicon, which looks ok according to http://www.colinkeany.com/favicon-checker/, but in general I think the logo isn't the best on dark themed browsers, so it may be worth having a specialized favicon version of the logo that looks good at the small size in all themes.

[codecov]

Codecov Report

Merging #245 (01e566d) into main (28d50b9) will not change coverage.
Report is 8 commits behind head on main.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #245   +/-   ##
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  Coverage   73.89%   73.89%           
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  Files          19       19           
  Lines        1149     1149           
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  Hits          849      849           
  Misses        300      300           

[jpreszler]

I took a stab at a specialized favicon that is a bit more distinctive in browser tabs (especially dark themed). I've added a white background, extracted the rocket and dag parts, and done some color and sharpness adjustments to help the contrast.

[jpreszler]

@drbenvincent this is ready when you get a chance.

[drbenvincent]

Nice! Sorry for the delay - I've had some newborn parenting challenges. I'll merge this and see how things go. We can always iterate on the logo or light/dark theme if needed, but this looks good already.