Porting guide should document transpiled porting

[dfabulich]

https://skip.tools/docs/porting/

As discussed here https://github.com/orgs/skiptools/discussions/382 this porting guide is entirely about porting via Skip's native/bridged mode.

The existing documentation should be updated to make it clear that it's talking entirely about --native modules.

There should also be a guide on how best to port modules in transpiled mode.