docs: Update theme to ReadTheDocs always (fixes regression)

[JST5000]

High Level Overview of Change

Since 2.2.0, the xrpl-py docs have been loading with a new theme. This brings it back to the previous ReadTheDocs theme.

Context of Change

It seems that the default theme changed without warning (as far as we saw):

Autogenerated theme code from before 2.2.0:
https://readthedocs.org/projects/xrpl-py/builds/21212455/

Autogenerated theme code from versions after/including 2.2.0:
https://readthedocs.org/projects/xrpl-py/builds/21889906/

In general you can check builds and autogenerated theming by going to this build page: https://readthedocs.org/projects/xrpl-py/builds/

1. Click an individual build
2. Click the "cat docs/conf.py"
3. Scroll down past the "auto-generated below here" comment
4. Look for where the theme is defined

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Documentation Updates

Did you update CHANGELOG.md?

• No, this change does not impact library users

Test Plan

Inspection and the docs build locally with the RTD theme ✅

The true efficacy will have to be determined after merging since this is a bug that only shows up in the production docs page, not in the local builds.

How it looks after the changes on ReadTheDocs:

[JST5000]

Thanks @mvadari for showing me how to activate a branch on ReadTheDocs :)

Added a screenshot for how it looks there, and updated the CONTRIBUTING.md with steps so other people can do that when modifying that config in the future.