Move the docs pipeline to Swift 6.0 toolchains

[czechboy0]

Motivation:

The docs check pipeline was pinned to 5.10, because the 6.0 and newer docc detected more issues in the docs, and pinning down was the short-term workaround.

Modifications:

• Stop pinning to 5.10, instead let it use 6.0 (and soon 6.1).
• Fix all the docs issues that 6.0 found.

Result:

• Gets the CI back to green when using 6.0 docc: https://github.com/apple/swift-distributed-actors/actions/runs/13986968195/job/39162333702?pr=1192

[czechboy0]

Merging over red CI - this fixed the docs and unit test pipelines. Will debug the remaining hanging 3 in a separate PR.