docs(eventing): page is stale and missing event source hierarchy

[Dennis-Mircea]

Summary

The eventing documentation page (docs/content/en/docs/documentation/eventing.md) has drifted significantly from the current code and is missing structural context that would help readers understand how the built-in event sources fit together.

What's stale

• The page repeatedly references CustomResourceEventSource, a class that does not exist in the repo (and as far as I can tell never has under that exact name).
• The #### ControllerResourceEventSource heading and its linked file path refer to the pre-2021 name. The class was renamed to ControllerEventSource in commit 86245ca6 ("refactor: renaming core classes and APIs"). The link 404s.
• The diagram at the top of the page labels the auto-registered SDK event source as "Custom Resource Event Source", which doesn't match the code.
• TimerEventSource is not mentioned anywhere on the page, despite being unconditionally instantiated for every controller in EventSources.<init> (line 44 of EventSources.java) and wired to the EventProcessor alongside the controller event source. It drives retry attempts, UpdateControl.rescheduleAfter(...), and the maxReconciliationInterval failsafe.
• The Registering Event Sources code snippet uses EventSourceInitializer<Webapp>, an interface that no longer exists. prepareEventSources is now a default method on Reconciler. The snippet also calls the reconciler WebappReconciler / Webapp while the actual sample file is WebPageReconciler / WebPage.
• The intro paragraph contains a broken sentence: "Event sources act as listen to events affecting these secondary resources..."
• The intro is one long paragraph mixing 4-5 distinct ideas (what event sources are, why you need them, non-Kubernetes use cases, efficiency).
• The "There are few interesting points here:" paragraph promises a list but is a single dense block.
• A "See related method" link in the Caching and Event Sources section points to line 146 of PerResourcePollingEventSource.java, which is a return; statement inside FetchingExecutor.run(), completely unrelated. The prose is describing getSecondaryResources(P primary) (around line 160 to 180 of the same file).
• Roughly a dozen inline links use the old GitHub org github.com/java-operator-sdk/java-operator-sdk instead of the current github.com/operator-framework/java-operator-sdk. They still redirect, but mixing the two orgs on the same page looks inconsistent.

What's missing

• No overview of the event source class hierarchy. Readers see a flat list of built-ins (InformerEventSource, PollingEventSource, etc.) with no indication of which classes share an abstract parent (ManagedInformerEventSource, ExternalResourceCachingEventSource) or where the EventSource interface and AbstractEventSource sit.