Use documentation from Apple's documentation bundle

[madsmtm]

Use the apple-doc crate I introduced in 0dd51d5 to parse the data that Xcode's documentation viewer uses (this is the same data as on https://developer.apple.com/documentation), and use that in header-translator to enhance the documentation we emit.

Fixes #309.

TODO:

• Finish the implementation, methods don't get this documentation yet, and links are currently to Apple's website instead of to the Rust item.
• Consider if there's some way we can include Apple's examples / tutorials, to truly make it unnecessary to want to go to Apple's website?
• Figure out the legal implications of this? We're already redistributing basically the entire Xcode SDK, this would just be taking and redistributing more of Xcode, but maybe there's a difference? Especially given that you cannot interface with Apple's platform without the SDK, and there is not other way to write them, whereas the documentation is maybe more legally protected?
  • I had a discussion with the Rust Foundation about this in September, but they're taking a long time to get back to me.
• Optimize this, cargo run --bin header-translator --release -- all now takes quite a bit longer.

---

As an example of how this would look, compare the documentation for NSVisualEffectView on Apple's website with what it would be in objc2-app-kit after this PR: