Write a better FAQ, living design doc

[tadasant]

As @connor4312 has pointed out, we are starting to get a lot of repeat questions that we have answers to scattered throughout docs and issues. The designs in the initial announcement are starting to grow stale, so we should have a better running document somewhere in version control.

This document should include:

• Overarching design goals / design principles to follow
• In scope / out of scope
• FAQ

[connor4312]

I think the overall scope and design goals can be better represented by you and others, but I'm happy to help contribute info about the package structure/openapi spec once there's a draft going

[toby]

@tadasant got this done in #134