Fixes sunpy/sunpy.org#339

I solved this issue in this repo instead of the webpage because this concerns citations of the core package, not the project more broadly.

This uses the ads package to automatically generate a list of every paper that has cited the SunPy. Here, I'm defining "cited SunPy" to mean either the 2020 v1.0 paper, the 2015 paper, or the JOSS paper. This is easily extended to others e.g. the Zenodo entries.

I'm then using sphinxcontrib-bibtex to generate the bibliography. Right now the formatting is a mess. The default BibTeX style does not look nice in our docs. There are ways to define custom styles with this extension (https://sphinxcontrib-bibtex.readthedocs.io/en/latest/usage.html#custom-formatting-sorting-and-labelling) so we could try that. The other option is to do away with bibtex all together and just format the list by hand using the info from the ADS search. However, if we're going to start using this extension anyway (see #7078), then it would be silly to not use it everywhere.

This is what things currently look like:

Some remaining todos:

• Add ADS token as a secret (currently, this PR will fail because there is no token)
• Fix formatting
• Sort citations by year instead of author name
• Add some sort of check that caches this result so we aren't making this query every time we build the docs