Feature: setup library user documentation

[chrisbc]

as we want this library to be usable by a wider audience it will need some decent documentation covering:

• [ ] usage
• [x] installation
• [x] user APIs (from source docs)
• [x] development and contributing
• [ ] testing
• [x] update the Readme (remove/remodel OG Chris 's aspirational stuff
• [x] scripts (use mkdocs.click)

Done when:

• [x] setup poetry for mkdocs (based on nzshm-documentation)
• [x] create index in mkdocs.yml
• [x] add content (as above)
• ~add github publish workflow~