[#311] Docs deps autogen sphinx plugin.

Added an `add_dependencies` plugin to the Sphinx build process that
parses the manifest files of the scanned backends and plugins and
automatically generates the documentation for the required dependencies
and triggered events.

This means that those dependencies are no longer required to be listed
in the docstring of the class itself.

Also in this commit:

- Black/LINT for some integrations that hadn't been touched in a long
  time.

- Deleted some leftovers from previous refactors (deprecated
  `backend.mqtt`, `backend.zwave.mqtt`, `backend.http.request.rss`).

- Deleted deprecated `inotify` backend - replaced by `file.monitor` (see
  #289).

docs/source/conf.py

@@ -52,6 +52,7 @@ extensions = [
     'sphinx.ext.githubpages',
     'sphinx_rtd_theme',
     'sphinx_marshmallow',
+    'add_dependencies',
 ]
 
 # Add any paths that contain templates here, relative to this directory.