Read the docs theme update

[hkershaw-brown]

Description:

This pull request updates the documentation them to sphinx-rtd-theme==3.1.0
The reason for updating is to use the readthedocs search.

Note for reviewers, the readthedocs search is only on the web (docs.dart.ucar.edu/version). Local builds will still have the same search as we have now.

The small downside I have found is that readthedocs doesn't instantly update the search index it builds, but I don't think this is a dealbreaker.

The setting
html_use_index = False
is to stop building an index page (D for data assimilation, O for awesome, etc.). We'd been using this to force results to show up in the previous theme's search. We may actually want an index page at some point in the future.

Fixes issue

fixes #1106

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update - the sphinx theme

Documentation changes needed?

• My change requires a change to the documentation.
  • I have updated the documentation accordingly.

Tests

Please describe any tests you ran to verify your changes.

Local builds, and
https://docs.dart.ucar.edu/en/rtd-theme-update/index.html

Checklist for merging

• Updated changelog entry
• Documentation updated
• Update conf.py

Checklist for release

• Merge into main
• Create release from the main branch with appropriate tag
• Delete feature-branch

Testing Datasets

• Dataset needed for testing available upon request
• Dataset download instructions included
• No dataset needed

[hkershaw-brown]

Also if you are building the docs locally note new requirements.txt for the new theme.

python -m venv new-docs-env
source new-docs-env/bin/activate
pip install -r $DART/guide/requirements.txt