Overhaul docs with new MkDocs-based site

[bradlarsen]

This overhauls the documentation for Nosey Parker, creating a new Material for MkDocs-based site.

The pages look like this at the moment.

Landing page:

Installation instructions:

Data model reference:

New listing of built-in rules:

Integrated full-text search:

[bradlarsen]

I'm still sorting out a hosting solution for these docs.

In the meantime, here are offline-generated docs for the current snapshot: site.tgz

[munntjlx]

Nice to see the docs they look beautimous!

[Coruscant11]

Nice one! Thanks for your work