Move documentation currently in README to the imap-processing docs

[bourque]

Through a previous slack convo, we decided it would be best to have all the documentation pertaining to the Data Access API all in one place, and decided that place should be the imap-processing ReadTheDocs page. Therefore, we should move the information provided in this repo's README to those docs, and then make it clear in this README that all of the supporting documentation for this repo can be found on the RtD page.