Also fix some Javadoc errors and ensure Javadoc (and source) jar generation is tested on CI.

* `Call` is the mechanism for request execution. It encapsulates a single request action either synchronously or asynchronously, and can support canceling.
* `CallAdapter` is a means of supporting a method return type other than `Call` (like RxJava's `Observable`) by adapting the `Call` instance into another instance. This change moves RxJava support into a sibling module which uses an adapter for supporting `Observable` types.
* `Response` represents the HTTP response along side the deserialized body. It allows access to both the converted body and lower-level items such as the header, response code, and underlying OkHttp request and response objects.

(cherry picked from commit 62903c4b)