Support DocC in Xcode 13

[mpdifran]

Apple introduced a new standard for method documentation in Xcode 13. We're already doing a pretty good job of documenting things, so there's only a few small tweaks we need to make to support it fully.

TODO:

• Build DocC on every build of the framework
• Update method doc formats to better take advantage of DocC
• Package DocC archives with the framework
• Create tutorials for common patterns in DocC

[mpdifran]

A good article on using DocC in open source projects https://www.jessesquires.com/blog/2021/06/29/apple-docc-great-but-useless-for-oss