Publish event-bus-kafka settings docs (annotations)

[timmc-edx]

Documentation on the configuration options for event-bus-kafka are currently spread between code comments (rST annotations on Django settings), in-repo doc files, and Confluence. Some of the information is needlessly duplicated, and is likely to get out of sync. We should publish the setting annotations (since they are authoritative) as is done in edx-platform, and then link to them for reference from any set-up guides.

• Set up ReadTheDocs for this repo
• Set up repo to publish setting annotations
• Deduplicate docs as necessary to point to the published annotations

We'll still want to have a sample config so we can highlight the essential settings, but this way we won't have to describe the details of all the settings in doc files. It will also allow us to avoid sending people into modules marked internal.