Add javadoc cross-linking for Fabric8 modules and external dependencies

[manusa]

Summary

Adds <offlineLinks> configuration to the maven-javadoc-plugin in both the release and javadoc-test profiles so that generated javadocs contain clickable hyperlinks to referenced types from other Fabric8 modules and external dependencies via javadoc.io.

All links use offline mode (-linkoffline) with static element-list files checked into doc/javadoc-links/. This avoids any network fetches during javadoc generation, which is critical because:

• Fabric8 SNAPSHOT versions don't exist on javadoc.io
• CI runners (GitHub Actions) cannot reliably reach javadoc.io

Cross-linked modules

• Fabric8 core: kubernetes-model-core, kubernetes-model-common, kubernetes-client-api, kubernetes-client, openshift-client-api, openshift-client
• External deps: jackson-databind, jackson-core, slf4j-api

Generated link examples

• KubernetesResource → javadoc.io/doc/io.fabric8/kubernetes-model-core/...
• ObjectMapper → javadoc.io/doc/com.fasterxml.jackson.core/jackson-databind/...
• TypeReference → javadoc.io/doc/com.fasterxml.jackson.core/jackson-core/...
• JDK types → docs.oracle.com/en/java/javase/11/docs/api/... (unchanged, via detectJavaApiLink)

Files changed

• pom.xml — <offlineLinks> and <detectOfflineLinks>false</detectOfflineLinks> in both profiles, plus <javadoc.links> property and user-agent header
• doc/javadoc-links/*/element-list — Static package lists for each linked artifact (9 files)
• Makefile — make generate-javadoc-links target to regenerate element-list files from source directories
• CLAUDE.md — Documents the new make target
• CHANGELOG.md — Entry under Improvements

Notes

• jackson-annotations is excluded because its Maven artifact version (2.20) differs from ${jackson.version} (2.20.0)
• Only element-list files are used (not package-list) — having both in the same directory causes JDK 11's javadoc tool to silently fail link resolution
• failOnError is left at the default (true) so real javadoc errors still break the build
• When external dependency versions are bumped, regenerate their element-list files by extracting from the new javadoc JARs

Fixes #1105

Test plan

• Build javadoc for kubernetes-client-api with -Pjavadoc-test — build succeeds with no network errors
• Verify Fabric8 cross-module links (KubernetesResource, UnmatchedFieldTypeModule) resolve to javadoc.io
• Verify Jackson links (ObjectMapper, TypeReference) resolve to javadoc.io
• Verify JDK links (Object, String, InputStream) still resolve to docs.oracle.com
• CI pipeline passes (no Error fetching URL failures)

🤖 Generated with Claude Code

[manusa]

@ash-thakur-rh I'm not sure if this is something we want to add or not.

It adds more complexity to the build but it also improves the help discoverability. These days this is tantamount since many are starting to rely on coding agents which require context discovery.

I'll leave this here waiting for feedback.