fix: add sphinx_rtd_theme to extensions in conf.py

[hkershaw-brown]

Description:

Add sphinx_rtd_theme so we use the latest version of the rtd theme.
This fixes the search function and flyout menu for readthedocs documentation
See issue #471 for discussion

Fixes issue

fixes #471

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

Tests

Please describe any tests you ran to verify your changes.
Built the docs, checked the search and the flyout menu (version choice bottom left) worked:
https://docs.dart.ucar.edu/en/doc-search-fix/README.html

compare to https://docs.dart.ucar.edu/en/v10.7.1/README.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

[mjs2369]

Approved. Fixes the search function and flyout menu.

I agree that using the sphinx_rtd_theme is the best way to do this.