docs: Hub plugin definition contributor guide

[pnadolny13]

It's not very clear to users how they can contribute to the hub plugin definitions. Since the hub relies on the community keeping it up to date with the taps/targets in the ecosystem, we should make this as easy as possible.

Right now we have a singer connector issue template but having a docs page that explains the process for adding and updating definitions. This could include other conventions and decisions we've made related to naming like #704 and how to handle things like our 2 taps for github that use the same API but have very different functionality github vs github-org.

[pnadolny13]

Potential overlap with #534

[tayloramurphy]

@pnadolny13 thanks for opening. It's definitely a weak spot for us. @afolson we should chat about this and figure out priority.