docs: initial docs work

[jsstevenson]

There's a lot of TODOs to... do, still, but this pins some rudimentary progress.

• Add docs skeleton, boilerplate
• First stab at setup instructions (they are a doozy)

Major remaining items in future PRs:

• some kind of big sources overview page
• description of harvest/transform procedure, references to VA/VRS
• more description of normalization