feature 253 readthedocs

[jprestop]

Pull Request Testing

• Describe testing already performed for these changes:
  
  Set up ReadTheDocs for MET and verified everything looked good here:
  https://metviewer.readthedocs.io/en/feature_253_readthedocs/
  Note that "latest" will point to "develop" until the release since this is the first version of documentation we've had for METviewer

• 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.

• Do these changes include sufficient documentation and testing updates? [No]

Redirection and documentation, including updating the Release Guide, will be done later.
dtcenter/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), 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]

Same changes as the ones for MET. And I double-checked the RTD docs and they look good. I approve.

[JohnHalleyGotway]

Same changes to enable RTD that Julie applied to the other METplus repos. I approve of these changes.