Extensions via entry points

- auto-loading of modules in the 'docker_registry.extensions' entry
  point group
- preliminary documentation
- define and encourage API on top of signals and config

This is an alternative approach to import_modules (#565) and namespace packages (#566).
Elaborate (now that config is no longer "segregated" on what the consumer needs to do - eg: "myconfig.extensions.mysuperextension").

Make it explicit we want people who are going to use the configuration that they must use a subnodes of ROOT.extensions.