Add a docs build

[craigds]

Add a docs build.

We shoudl have a docs/ dir containing a bunch of restructuredtext, and build them using sphinx or similar into a readthedocs site.

To start with this should be the same usage docs as exposed by sno help, but it can easily grow from there. We have a bunch of assorted docs in the wiki and markdown files in the codebase, that should be included in a docs build.

Having them in a github wiki is okay but:

• it's hard to keep doc changes in sync with code changes (i.e. they're not included in PRs)
• they're not available from the command-line
• there's no possibility of auto-generating any documentation; it's all manual point-and-click editing

[rcoup]

I agree 👍

• they're not available from the command-line
• there's no possibility of auto-generating any documentation; it's all manual point-and-click editing

FWIW, you can access Github wikis as a repo and it supports various flavours of structured text: https://docs.github.com/en/github/building-a-strong-community/adding-or-editing-wiki-pages