Point new contributors to other pieces in the RTD ecosystem

[agjohnson]

The core team has decided that the code for Read the Docs is not the best place to send new contributors. We've found it's really hard to point contributors to digestible work, and setup is already complex, and scheduled to become more complex. There are a number of areas where contributors can get started with less trouble:

• Sphinx extensions
• Our theme
• Eventually we want to have digestible extensions and integrations, which are maybe external from our central code

We should update our contributing materials to reflect this decision

[ericholscher]

Small version of this is done! Not sure if we need more here -- I think we can probably close it.

[humitos]

I'm fine closing this issue. We can point people to addons eventually as well since those are more self-contained and may not require the whole Read the Docs development environment to work on them.

However, we are in a transition now to redesign the UI so may not be a good moment to point people there right now.