398 expand documentation for extending geoips with plugins

[evrose54]

Reviewer Checklist

• [ ] Required existing tests pass (ie full_test.sh, others as appropriate)
• [ ] NO REQUIRED unit tests (explain why not required)
• [ ] NO REQUIRED integration tests (explain why not required)
• [ ] Required documentation added for new/modified functionality
• [ ] Required release notes added for new/modified functionality
• [ ] NO REQUIRED updates to other repos (explain why not required)

Related Issues

fixes NRLMMD-GEOIPS/geoips#398

Testing Instructions

Run ./docs/build_docs.sh . geoips html_only, download the build/sphinx/html directory and place the index.html file in your browser to view the new documentation.

Summary

Will be updated once this PR is ready for review.

[evrose54]

Still in progress but I wanted to put this up here in case any of you had thoughts on this. Also, I think my main is out of date, which is why some files (like output checkers and single_source) have updates that I did not make. I've tried git fetch and git pull on main and that isn't fixing it currently.