Add documentation for content providers

[seadowg]

We should add docs (probably to https://docs.getodk.org/launch-collect-from-app/) for how other apps can query forms and instances in Collect. For the moment, the intention is to remove mutation methods from the content providers, so we shouldn't document those. Things to include (and potentially improve in Collect):

• [ ] How to get the list of forms and instances
• [ ] How to get an individual (or filtered list) form or instance based on db ID or form id/version (we should probably not document or support any other kinds of query)
• [ ] How to read data from the returned cursor (good to think about the best way for an app developer to keep the column names etc up to date)
• [ ] How to link the returned forms/instances with filling forms etc