Work around scaladoc JIT NPE on JDK 25

[ghostdogpr]

Root cause

The publish job (and, since #159 added opentelemetry/doc, the docs job) fails with a scaladoc crash:

java.lang.NullPointerException: ... the return value of
"dotty.tools.scaladoc.translators.SignatureBuilder.content()" is null
    at ...SignatureBuilder.termParamList

This is scala/scala3#24183: on JDK 25 the C2 JIT miscompiles a combinator in scaladoc's SignatureBuilder, so content() returns null once that method gets JIT-compiled. That only happens after the method is warmed up by several doc runs in one JVM, which is why it hit a random cell and only in the many-cell ci-release / docAll runs, not standalone. Started when CI moved to Temurin 25. Fixed upstream in Scala 3.9.0 (scala/scala3#25779).

Confirmed locally: with C2 disabled (-XX:TieredStopAtLevel=1) or the offending class excluded from JIT, docAll goes from reliably crashing to 0 crashes.

Changes

• .jvmopts: exclude dotty.tools.scaladoc.translators.SignatureBuilder from JIT. sbt reads .jvmopts on every invocation, so this covers both the docs and publish jobs (and local doc/publishLocal) with C2 left on everywhere else. Harmless where scaladoc isn't loaded.
• build.sbt: remove the doc-task serialization from Serialize scaladoc during publish and fix opentelemetry doc links #159 (scaladocTag, concurrentRestrictions, the Compile / doc tag). It was based on a wrong concurrency hypothesis and had no effect — the crash reproduces with doc tasks run strictly serially.

Verification

• sbt docAll on a clean fresh JVM: 0 crashes across two trials (previously ~reliably crashed), and the docs-gate warning grep passes.
• Confirmed via RuntimeMXBean that the .jvmopts flag actually reaches the sbt JVM.
• scalafmtSbtCheck passes.