Improve documentation

[lightsprint09]

No description provided.

[lightsprint09]

Could be useful: https://github.com/SwiftDocOrg/swift-doc#github-action

[heckj]

Since Apple made DocC available (and I was already familiar with it due to past work experience), I made a stub PR that would start to flesh out the docs using that setup and the code documentation already in place: automerge/automerge-swift#43

DocC currently doesn't enable an easy "plop into GitHub pages" kind of flat HTML hosting scenario, but that's actively being worked on - and when available, I'd be happy to help set that up (including to the GitHub pages associated with this repo if you like). That said, the current DocC pieces do integrate nicely into Xcode for anyone wanting to work on or use automerge-swift as a dependency in their project.