Add more detail to Figma setup

What does this MR do?

Towards #53 (closed). Expands on the developer documentation. The goal for this MR is to provide just enough info for a brand new developer to come into the project, contribute, and publish a release to the Figma plugin directory. Due to it being a mostly manual process, the instructions get a little verbose...

Note that in the future, we will hopefully trim a few of these steps down with additional automation, but the core of the release process will remain the same.

Checklist

• [-] Added tests for new functionality