docs: Expand Configuration>Reporters section.

[ryneeverett]

• Diagram reporter hierarchy. This also serves to list the available reporters.
• More explicitly distinguish between common options and reporter-specific options.

I would note that I initially tried the sphinx.ext.inheritance_diagram directive but I found the resulting graphviz image buggy, hard to read, and also useless for the man page. So I implemented an extension myself and tried to mimick the unix tree command format, mostly due to lack of creativity.

   html
   text
   ├───stdout
   ├───email
   ├───ifttt
   ├───text
   │   ├───pushover
   │   └───pushbullet
   ├───mailgun
   ├───telegram
   ├───slack
   │   └───mattermost
   ├───discord
   ├───xmpp
   ├───prowl
   └───shell
   markdown
   └───matrix

[ryneeverett]

Looking back at this, I just noticed that WebServiceReporter doesn't have a __kind__ attribute.