Use Docc for documentation

[Lukasa]

Motivation

Documentation is nice, and we can help support users by providing useful clear docs.

Modifications

Add Docc to 5.6 and later builds Make sure symbol references work Add overview docs

Result

Nice rendering docs

[Lukasa]

Uh, this is new. @yim-lee any idea what's happened to SwiftPM here? It seems to have generated a test file that emits a compiler warning, which we choke on because we use warnings as errors.

[Lukasa]

Oh dang it, I know what's happening: this is because there are no tests on Linux. Ok, I'll add a dummy test.

[yim-lee]

🎉