Update AppHost eventing docs to use typed helper APIs

[Copilot]

AppHost eventing docs still showed older Eventing.Subscribe<T> patterns in places where typed helper APIs now exist. This updates the eventing guidance and related snippets to prefer the named helpers while keeping generic subscription examples only where the lower-level API is the point.

• AppHost eventing page

  • Clarified the builder-level helper methods available for AppHost lifecycle events.
  • Added API reference links for the helper surface.
  • Updated the resource stopped example to use the typed helper pattern.

• Related doc snippets

  • Replaced legacy subscription examples with helper methods where a direct helper exists:
    • OnBeforeStart
    • OnAfterResourcesCreated
    • OnInitializeResource
    • OnConnectionStringAvailable

• Release notes and examples

  • Updated existing release-note and architecture snippets so they align with the current eventing guidance and preferred API shape.

// Before
builder.Eventing.Subscribe<BeforeStartEvent>((e, ct) =>
{
    return Task.CompletedTask;
});

// After
builder.OnBeforeStart((e, ct) =>
{
    return Task.CompletedTask;
});