docs generate RPC task (#1781)

[beckjake]

Fixes #1781

Implement docs generate for the RPC task. I went with docs.generate for the RPC call name, since . is allowed and it seems like a nice namespace separator. That way we can do dbt source snapshot-freshness -> source.snapshot-freshness in the future, and the mapping is reasonably straightforward.

[drewbanin]

It doesn't look to me like the docs.generate task creates a manifest.json after compilation -- can you double check that and add a test for it?

Otherwise, this LGTM

[beckjake]

It doesn't look to me like the docs.generate task creates a manifest.json after compilation -- can you double check that and add a test for it?

ok. the issue is that we do generate a manifest.json but only during the RPC server's compilation, not during catalog generation.

[drewbanin]

looks great -- ship it!