Docs: Python API: Switch theme to Furo

Since moving from the Wiki we have used the `sphinx_rtd_theme` which has worked well but has become a little dated and has had slow development. This changes to use "Furo" instead.

[Furo](https://github.com/pradyunsg/furo) is a lightweight theme that is responsive and has native dark mode support.

Dark mode is one of the major enhancements but this change also brings:

- Content is slightly larger making text and images easier to view
- Site navigation and page navigation is split into two navigation trees. The site navigation is on the left and page on the right if the page has multiple headings.
- Return to top button
- Content is centered on the screen which helps with wide monitors
- No more breadcrumbs
- Remove google analytics

So far the caveat that I have noticed with this is slower compile times and larger pages, this is due to the better sidebar navigation.

This change also brings a lot of simplifications to customizations that we made to the `sphinx_rtd_theme`. There is still likely room for improvement in the future.

Pull Request: #119684

doc/python_api/requirements.txt

@@ -9,4 +9,4 @@ requests==2.31.0
 
 # Only needed to match the theme used for the official documentation.
 # Without this theme, the default theme will be used.
-sphinx_rtd_theme==1.3.0rc1
+furo==2024.1.29