Fix sync-content workflow for apache/pulsar gradle build

[lhotari]

Summary

• apache/pulsar master switched to a Gradle build, breaking the sync-content composite action which still ran mvn install. Maintenance branches still use Maven and reuse the same Python entry points at release time.
• Add a pulsar_build helper that detects gradle vs maven from the pulsar checkout layout, exposes build-system-specific classpath / swagger paths, and runs ./gradlew assemble :distribution:pulsar-server-distribution:exportClasspath :distribution:pulsar-shell-distribution:exportClasspath on a fresh gradle checkout.
• Route all five reference/CLI doc generators and swagger_generator through the detector so the same scripts work on master (gradle) and on maintenance branches (maven, behavior unchanged). swagger_generator skips with a warning on gradle since apache/pulsar master has no swagger task yet; the mvn -Pswagger path is preserved for maven.
• Make site_uploader.execute()'s head_sha optional. site-updater.py had stopped passing it after the .publish-ref change, which left the sync workflow latently broken; head_sha=None now skips the .publish-ref write that is only meaningful for asf-site-next.
• Rewrite .github/actions/sync-content/action.yml to use gradle/actions/setup-gradle@v5 and ./gradlew assemble exportClasspath instead of mvn install.
• Drop PULSARBOT_TOKEN from .github/workflows/ci-sync-content.yml and grant the default GITHUB_TOKEN contents: write so the scheduled workflow can push to main.

Test plan

• Trigger CI - Synchronize Content via workflow_dispatch and confirm the gradle build + doc generators complete, that swagger generation skips with the expected warning, and that the resulting commit pushes to main using the default GITHUB_TOKEN.
• Run bin/reference-doc-generator.py --master-path=<maintenance-branch-checkout> locally against a maven branch and confirm behavior is unchanged (classpath read from target/, mvn -Pswagger fallback intact for rest-apidoc-generator.py).
• Run the same generators against an apache/pulsar master checkout and confirm pulsar_build.ensure_built() produces the gradle classpath/jars and the reference + CLI docs generate.