Complete reference for IDL, DSL, and sources and sinks

[lanthias]

User Story

• As a user, I want an overall reference on supported commands, supported sources and sinks, and API documentation so I can build things.

Acceptance Criteria

• API reference for the CLI
• Module structure
• IDL and DSL reference
• Supported Integrations (e.g. Sources and Sinks)

[lanthias]

This still needs triage for sources, sinks, idl, and dsl.

[lanthias]

@feuerbach helpfully added documentation around sources and sinks in #19

[UnkindPartition]

The language reference is now merged (#29), closing.