ReadTheDocs build has warnings when successful

[pllim]

The last successful log of ReadTheDocs showed 27 warnings. These warnings should be fixed; maybe fixing them would also fix problems like images not showing properly (screenshot below):

Bonus: You can set up ReadTheDocs YAML configuration to turn warnings into failure, which would prevent the warnings from cropping back in the future.

Also see #50 and #53

https://github.com/astropy/astropy.github.com/pull/491#issuecomment-1215349065