get_psm3, 0.9.0 whats new doc fixes

[wholmgren]

• Closes #xxxx
• 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`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• 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.

[wholmgren]

@kanderso-nrel The "Data File Format" section of https://developer.nrel.gov/docs/solar/nsrdb/psm3-tmy-download/ contains a broken link to a PDF documentation of the format. This PR brought to you by my surprise that the TMY data from the NSRDB API (or at least pvlib's get_psm3 function) returns data on the half hour instead of the hour (like the old files).

[kandersolar]

@kanderso-nrel The "Data File Format" section of https://developer.nrel.gov/docs/solar/nsrdb/psm3-tmy-download/ contains a broken link to a PDF documentation of the format.

Thanks. If you're still after that PDF in the meantime, there's a copy of it on the wayback machine -- see link here: NREL/developer.nrel.gov#196