Add documentation for gears

[rteabeault]

Gears seem like the primary way of extending motherbrain. It would be great to get some documentation on:

• the architecture
• what is possible
• some examples of how to write them and how to call them in the motherbrain.rb

[capoferro]

The original idea behind gears was to make them the method of extending motherbrain, though at the moment they're not exactly user friendly and there's not been any features that require more gears recently except the DynamicService work that are sort of a strange example of a gear as it is. I'll leave this open as a reminder to document them at some point. Hopefully soon.