Replace custom doc archive merging with Swift 6 docc multi-target support

[finestructure]

Swift 6's docc supports generating docs for multiple targets. We should explore how this could replace our custom archive merging.

• Check what a multi-target doc archive looks like rendered - does it have a target switcher?
• Does this conflict with our existing target switcher?
• Could/should we keep both switcher such that both <Swift 6 generated docs and Swift 6 generated docs can be hosted?

[daveverwer]

It depends what it looks like, but I'd say ideally we would remove our target switcher for anything pre-6.0 but keep the version switcher in tact.