Use `appledoc` to build the documentation.

This also closes #822. Note that you won't be able to build the docs from `master` because of the weird `-_contents` accessor in `QSCatalogEntry` (fixed by #1601, so you can only use it on that one for now).

The `./Tools/build_docs.sh` script builds HTML documentation in `./Docs` by default, and an Xcode docset if you pass it `-d`.