docs: docs rewrite

[daler]

First pass of rewriting documentation, currently focusing on just the index page to make it as informative as possible for new users without being overwhelming.

Highlights:

• includes new .. details:: directive and front-page diagram
• removed the last vestiges of circleci (docs building was the only thing running on it anymore) and ported to GitHub Actions
• user documentation is all on the first page, deferring to either .. details:: blocks or FAQs
• includes channel_priority strict recommendation (from docs: recommend strict priorities #776 )
• updated requirements (as identified by building autosummary docs) and fixed various formatting issues in docstrings

See additional notes for where this is going as a whole.

Edit: now includes fixes #779, #778, #777 (thanks @siebrenf)

[daler]

Closing in favor of #781