DOC: Design drafts to assist with next-gen pandas internals discussion
Sprinted on my accumulated design ideas. There's more to do, but I'm going to take a break for a bit. 

I published this as a sphinx site temporarily to https://wesm.github.io/pandas2-design/ so it's easier to read. This needs to be a team effort, so depending on where the discussions go perhaps we can build a set of documents that we can make PRs to and discuss specific matters either in GitHub issues or on the mailing list. Let me know what everybody thinks!
