Issue #15608: Improve socketserver module documentation

* Add headings for each concrete and mix-in class and list methods and
  attributes under them
* Fix class and method cross references
* Changed RequestHandler to BaseRequestHandler and added class heading
* Pull out Stream/DatagramRequestHandler definitions
* Reordered the request handler setup(), handle(), finish() methods
* Document constructor parameters for the server classes
* Remove version 2.6 not relevant for Python 3 documentation