feature_15_readthedocs

[jprestop]

Pull Request Testing

• Describe testing already performed for these changes:
  
  Set up ReadTheDocs for MET and verified everything looked good here:
  https://metexpress.readthedocs.io/en/feature_15_readthedocs/

• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
  
  Review the "Files Changed" and look at the webpage above and let me know if you see anything that needs modifications.
  I do not believe I have the authority to merge here, but I am not sure. Could you please also merge it?

• Do these changes include sufficient documentation and testing updates? [No]
  Redirection and documentation, including updating the Release Guide, will be done later.
  ReadTheDocs: Set up gh-pages redirection and update Release Guide METplus#836

• Will this PR result in changes to the test suite? [No]
  

Pull Request Checklist

See the METplus Workflow for details.

• Complete the PR definition above.
• Ensure the PR title matches the feature or bugfix branch name.
• Define the PR metadata, as permissions allow.
  Select: Reviewer(s), Project(s) (there was no appropriate project for me to select), and Milestone
• After submitting the PR, select Linked Issues with the original issue number.
• After the PR is approved, merge your changes. If permissions do not allow this, request that the reviewer do the merge.
• Close the linked issue and delete your feature or bugfix branch from GitHub.

[JohnHalleyGotway]

These are the same changes that @jprestop has made to facilitate RTD in the other METplus repos. I approve of these changes.
Note that these settings PREVENT the publication of a PDF version. @ksearight is coordinating with Bonny of the formatting of the PDF.
@mollybsmith-noaa this merges the changes into the development branch. Recommend that you also merge them into the master (main) branch eventually.