Embedded documentation for the Y-CRDT swift overlay, and hosting in Swift Package Index

[heckj]

I suspect it might be awkward to include documentation with the scaffolding generated by uniffi, but I'd very much like to include the relevant markdown documentation, akin to what Bartosz created for the 0.16 Rust crates release.

Primary goal being to show the basic usage of the code, and to build that up as we complete the Yrs overlay. This is mostly something that gets added to any swift overlay code, in its source and as an additional directory in that same Package.swift location. The result being two-fold:

• After you import the code as described in Consumption goal for the library  #8, in Xcode you can run "Product -> Build Documentation" and have the basic docs for this library included locally in Xcode.
• As we make releases available for others to use, register the overlay with Swift Package Index so that the package is both in the index, and for which they'll host documentation (very similarly to how Crates hosts docs).

[heckj]

With the 0.1.0 release, I've submitted the package into the Swift Package Index, but I'm not certain that documentation will be easily hosted with them as well. When it's fully processed and available, I'll double check and see what next steps are to get it up somewhere, if the SPI hosting isn't automagic for us.

[heckj]

https://swiftpackageindex.com/y-crdt/yswift

[heckj]

Docs sorted, Swift Package Index updated and linking to correct docs.