Serialize scaladoc during publish and fix opentelemetry doc links

[ghostdogpr]

Problem

The publish job (sbt ci-release) fails with a scaladoc crash while documenting the clientKyo (Scala 3.8.4) cell:

Caused by: java.lang.NullPointerException: ... the return value of
"dotty.tools.scaladoc.translators.SignatureBuilder.content()" is null
    at ...SignatureBuilder.termParamList
(clientKyo3_8_4 / Compile / doc) java.lang.reflect.InvocationTargetException

The identical clientKyo3_8_4/doc task succeeds in the docs job of the same run and locally. dottydoc is not thread-safe, and ci-release runs packageDoc for every published cell in parallel in one JVM (far more than the docs gate's docAll), intermittently tripping the race.

Changes

• Tag every Compile / doc task and cap the tag at 1 concurrent, serializing scaladoc during publish.
• Drop two unresolvable [[OpenTelemetry]] doc links (external type dottydoc can't resolve) in favor of monospace.
• Add opentelemetry/doc to docAll so the docs gate covers that module (it previously slipped past the gate).

Verification

• sbt docAll passes with no Couldn't resolve / warning lines.
• Build reloads cleanly; the tagged doc task resolves on all cells.