Add doc on porting code to non-Apple platforms

[finagolfin]

I wanted to add a documentation page showing common patterns in porting Swift packages to Android, then @compnerd pointed out that we have no such doc for any platform now. Since all non-Apple platforms will need similar changes, like adding the relevant overlay imports, along with a few platform-unique needs, perhaps we can put together a single doc page for all these new platforms.

@Joannis, @al45tair, @kateinoigakukun, @marcprux, let me know what you think about collaborating on such porting doc, including platform-specific info for linux servers, Musl, Wasm, and Android. I think this would help us move cross-platform compatibility up from the 20-40% currently reported on the package index.

[al45tair]

I think this is a good idea. @heckj, where should we look to add this? Probably the thing to do is to raise a PR and collaborate on writing up the necessary information.

[kateinoigakukun]

Yeah, great idea. We currently have a brief page in our documentation site, but it would be nice to have a comprehensive one alongside other platforms!

[heckj]

That's a really good question. At the moment, the best place would be to land it as an article in swift.org, but that's suboptimal for quite a few reasons, most of which resolve around a issues with findability and linking - as there's no clear place to connect with through the redesign.

There is a growing collection of articles about the Swift language itself, but it's broken up a bit between the migration guide (https://github.com/swiftlang/swift-migration-guide), what's settled in the Swift repository now (content in https://github.com/swiftlang/swift/tree/main/userdocs, treated as a DocC catalog and published to https://docs.swift.org/compiler/documentation/diagnostics/), and what's included around the TSPL (https://github.com/swiftlang/swift-book/)

The space that we need here is loosely a collection place for "guides to writing in Swift" that aren't the reference content. In the earlier docs proposal, I had a space for "swift language guides", but that lacked a clear set of reviewers. I think it something along those lines could make a lot of sense with the platform group as reviewers, potentially a collection of "how to write Swift for..." articles that members of the Platforms team could help review for accuracy.

[Joannis]

This would be really helpful!