Testing documentation build in Azure Pipelines

[CameronTStark]

• Closes add sphinx documentation build to CI #771
• I am familiar with the contributing guidelines
• Tests added
• Updates entries to docs/sphinx/source/api.rst for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels and Milestone are assigned to the Pull Request and linked Issue.

This PR builds the docs on Azure PIpelines. It passes with many warnings and I'm not sure if this is due to my sphinx-build call, the environment or a problem pvlib has been having with documentation on an ongoing basis. I'd appreciate others with more documentation experience to chime in if not someone who's already done this in another repo.

The magic starts on line 31. Thanks!

[wholmgren]

The full doc build requires most (all?) of the [optional] requirements. From a very quick look, I think that's the cause of many of the warnings.