Java Docs

[kraftp]

First draft of Java docs. TODOs:

• Add the example apps + quickstart after the API is finalized.
• Document scheduled workflows. Can't figure out how the current API works.
• Figure out if we want users to call singleton instance methods or static methods bound to the singleton, then update docs appropriately.
• A few references are incomplete as the interfaces need changing (listWorkflows, WithQueue)
• Need recommendations for getting started with Postgres. Will there be a CLI?