Set up sphinx gallery; add clearsky.reno example

[kandersolar]

Description

This PR:

• Sets up an example gallery using sphinx-gallery: link
• Adds a short example of clearsky.reno: link
• Configures function-level docs pages to have a "mini-gallery" at the bottom listing examples using that function: link

Checklist

• Closes Sphinx docs don't show example usage #125
• Added new API functions to docs/api.rst
• Clearly documented all new API functions with PEP257 and numpydoc compliant docstrings
• Adds description and name entries in the appropriate "what's new" file
  in docs/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`).
• Non-API functions clearly documented with docstrings or comments as necessary
• Added tests to cover all new or modified code
• Pull request is nearly complete and ready for detailed review

[kandersolar]

Ok, this one is ready now too. Right now the example makes a network request; maybe we should change that to use a small file in pvanalytics/data?

[cwhanse]

+1 to adding a file to pvanalytics/data. Getting remote data for tests is a frequent maintenance issue. OK with me to leave the code for getting the data in the example as a comment.

[cwhanse]

This is great, thank @kanderso-nrel