[WIP] Adds a summary of the ORA2 data schema.

- includes a diagram of how the tables relate to each
- Note: diagram doesn't include the anonymous-id-to-real-id mapping table
- includes a summary of the conceptual pieces behind the big pile of tables.
- does not describe the AI-related tables in detail: AI grading isn't ready
  for broad use, and those tables aren't included in exports yet.

Note: I haven't tried to figure out how to actually build all the docs, so I'm not sure I got the linking right. I do think that this is enough of an overview to get people started analyzing the data, though certainly we could add more details.  The corresponding exporter PR is https://github.com/edx/analytics-exporter/pull/51