Add new docs theme, and restructure

[vkbo]

Summary:

This PR replaces the plain read the docs theme of the documentation with a theme called furo. The theme has a few benefits over the standard RTD theme. It is wider, and uses the browser space better. It has a contents table if the browser window has the room. It has a dark mode. The overall design is cleaner,

In this PR the documentation is also restructured a bit. I'm trying to gradually increase the information and details as you read further instead of dumping a lot of detail right away. I've converted a section of the Introduction chapter into a Key Features chapter. This is then followed by a quick run-through of all the main components.

A preview is available here at least until the PR is merged.

Related Issue(s):

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed