Adds stingray's logo and favicon in the documentation

[devanshshukla99]

As highlighted in #705, the documentation contains astropy's logo at the top left and as the favicon. This PR replaces those with the stingray's logo, fixes #705

The favicon can be replaced easily by overriding html_favicon in conf.py, unfortunately, the same is not true for the logo in the top left, since it's hard coded into astropy_sphinx_theme at https://github.com/astropy/astropy-sphinx-theme/blob/main/astropy_sphinx_theme/bootstrap-astropy/static/bootstrap-astropy.css?plain=1#L253-L255

The current implementation works by overriding the css propery using custom.css and adding the 32x32 stingray's logo in base64 form.

Rendered docs:

[codecov[bot]]

Codecov Report

Merging #707 (a9d62a6) into main (fb98a57) will not change coverage. The diff coverage is n/a.

:exclamation: Current head a9d62a6 differs from pull request most recent head 300dd93. Consider uploading reports for the commit 300dd93 to get more accurate results

@@           Coverage Diff           @@
##             main     #707   +/-   ##
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  Coverage   97.21%   97.21%           
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  Files          42       42           
  Lines        7797     7797           
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  Hits         7580     7580           
  Misses        217      217           

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[matteobachetti]

Thanks @devanshshukla99 !