feature_readthedocs_v3.0

[jprestop]

Pull Request Testing

• Describe testing already performed for these changes:
  

Set up ReadTheDocs for METplus and verified everything looked good here:
https://metplus.readthedocs.io/en/feature_readthedocs_v3.0

• 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 updates, ensuring that no errors or warnings exist in the build of the documentation? [No]

Redirection and documentation, including updating the Release Guide, will be done later #836

• Do these changes include sufficient testing updates? [Yes]

• Will this PR result in changes to the test suite? [No]
  
  If yes, describe the new output and/or changes to the existing output:
  

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.

[georgemccabe]

These changes are similar to the PR for v3.1 (#864) with the addition of an ignore entry for the generated doc files and adding a requirements file for the repo. I approve.