Update Docs Theme to use `sphinx-book-theme`

[oliverholworthy]

Update Docs Theme to use sphinx-book-theme instead of sphinx-rtd-theme

Details

Existing Functionality

Checking some things are still working as before:

• [X] Google Analytics Tag still added to header with analytics ID
• [X] Version Selector

New Features

• Added Ecosystem section with links to other Merlin packages
• Updated theme with NVIDIA colours adapted from the Omniverse docs

Before

After

[github-actions[bot]]

Documentation preview

https://nvidia-merlin.github.io/core/review/pr-330