Correct serializer loading and supervision documentation

[Aaronontheweb]

Configured custom serializers use the .NET constructors selected by alias configuration, with no parameterless fallback. Correct the Serializer XML and serialization article to describe that loading path and SerializationSetup.

Document legacy BackoffSupervisor immediate restarts versus the delayed Backoff.OnFailure path, retry boundaries and reset behavior. Correct the router article to describe the current default supervisor strategy.

Fixes #5065
Fixes #6976
Refs #4702
Refs #647

Validation of the combined documentation candidate:

• Release builds of Akka, Akka.Persistence, Akka.Streams, and Akka.DistributedData pass with warnings treated as errors; all 33 API approval tests pass.
• Roslyn comparisons confirm identical C# tokens and non-XML trivia; six verifier controls pass. No runtime, public API, signature, or nullability changes.
• Two independent GPT-6.1 Sol reviewers checked this batch against implementation; all identified corrections were addressed and rechecked.
• Markdown lint, Slopwatch, and the staged DocFX website build pass. Existing article anchors and maintained code samples are preserved.
• Fresh API metadata for the four affected projects introduces no reference warnings. Metadata generation with warnings as errors still fails on the same pre-existing ActorSystem.cs reference to ConfigurationFactory.Load() as the baseline. Other API metadata was cached for cross-reference resolution during the website render.