support docs generation in the rpc server

[drewbanin]

Describe the feature

The rpc server should support the generation of documentation artifacts (manifest, catalog, run_results, index.html) via an rpc method. This method can be named something like docs:generate or docs_generate, and the scheme we use should extend well to other subcommands like source snapshot-freshness. The artifacts generated by this method should land in the configured target/ directory.

This method should support a parameter that specifies the --no-compile functionality.

We should not yet implement a method to serve artifacts from the target/ directory, though we may prioritize that in a future issue.