Remove Jazzy doc generation

[glbrntt]

Motivation:

Docs are generated by and hosted on the Swift Package Index. We no longer need the script to generate docs via Jazzy.

Modifications:

• Remove the generate_docs script
• Remove Jazzy from the Dockerfile but keep Ruby; it's used for generating test manifests.
• Remove SwiftFormat from the Dockerfile; we don't use it.

Result:

Fewer unused things.