Document input/output plugins

[hexedpackets]

Several plugins are not listed in the ffwd documentation. A bare minimal page for each showing that the plugin exists and what its configuration options are would be useful. I think the undocumented plugins are:

• [ ] carbon
• [ ] kafka
• [ ] opencensus
• [ ] riemann
• [ ] signalfx
• [ ] template
• [ ] generated
• [ ] debug
• [ ] noop