Document the Inventory REST API

Users shouldn't necessarily be needing to know what the REST API is, so we don't need this as part of our official and supported docs, but we should still document the api for developer knowledge.

This will make it easier for developers to know what API exists, and options are available and how to use things (eg for working on the ruby gem, working on test cases, making sure we don't accidentally add in a redundant endpoint when another can do the same thing, etc)