Update and cleanup data documentation

[dfsnow]

• [x] #7
• [ ] #6
• [x] #5
• [x] #2
• [ ] #3
• [ ] #4

[jeancochrane]

@dfsnow It seems to me that most of these docs should probably live in the data catalog (https://github.com/ccao-data/data-architecture/issues/10). If you're aligned, I'll deprioritize this issue and focus on the catalog for now, and then revisit this once we have a better sense of which documentation doesn't make sense to keep in the catalog.

[dfsnow]

@jeancochrane Works for me!