Optimize docs build

[davidme-stripe]

Summary

• We were running SourceKitten multiple times over each module. Reduce that to only run once on all modules before building the docs.
• Use incremental builds where possible.
• Upload the produced docs as an artifact in Bitrise.
• Switch to Xcode 13 for now, as it's what we use to deploy the docs and Xcode 14 isn't outputting correct attributes for @_spi classes.

Motivation

Reduce docs build time from 15m to 9m, prepare for deploy script changes.

Testing

CI, on Xcode 13