DOC: added explanation of each of the documentation sections
This is meant to help contributors understand what information belongs where.

- [ ] ~~closes #xxxx~~
- [ ] tests ~~added /~~ passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] ~~whatsnew entry~~

@afeld do you want to merge master here? This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. > @afeld do you want to merge master here?

@afeld can you do this and respond to https://github.com/pandas-dev/pandas/pull/41689#discussion_r641739368 Thanks for the PR, but it appears that this PR has gone stale. Close due to inactivity but if you're interested in continuing let us know and we can reopen.