Migrate to DocC Combined Documentation

[orchetect]

DocC currently has an experimental combined documentation feature that can combine docs for multiple package targets and synthesize a landing page to navigate them using navigation hierarchy.

I spent some time refactoring TimecodeKit's documentation to adopt this feature and it works fairly well.

Until now, MIDIKit has used a rather complex set of manual scripts I built in order to merge docs from all of its targets together and perform the required structural and syntax changes required to accommodate it. This new DocC feature makes most, if not all, of that a 1st-party feature and would make the scripts redundant.