docs: Document custom serializable exceptions

[loopassembly]

Summary

• Add guidance for custom Dart exception classes that should be serialized to the client.
• Document that custom exception classes must implement SerializableException.
• Clarify that external/shared exception classes still need an extraClasses entry so generated serialization can reconstruct them on the client.

Verification

• npx --yes --cache ./.npm-cache markdownlint-cli docs/06-concepts/04-exceptions.md
• git diff --check

I did not run a docs build because serverpod_docs/AGENTS.md says not to run build commands locally.

Closes serverpod/serverpod#3421

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds documentation explaining how to create and register custom exceptions that can be safely serialized and sent from Serverpod server to client.

Changes:

• Documented SerializableException implementation for shared Dart exceptions
• Documented registering shared exceptions via extraClasses in config/generator.yaml

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[loopassembly]

Thanks for the review notes. I checked these against the source and nearby docs examples. SerializableException already implements Dart’s Exception interface, and the repo consistently documents extraClasses package/class entries as plain YAML scalars.

I kept the example aligned with that source contract and repo style, then reran the checks. The latest title, markdown-lint, and Docusaurus test-build checks are all green now.

[Swiftaxe]

Very neat PR. Just added a small suggestion.

Thank you for this contribution @loopassembly!

[Swiftaxe]

Nit: The very first sentence, "Use the 'exception'..." is a repitition of above under ## Serializable exceptions. Also it seems to contain a line break after 'new'. Suggestion to cut the sentence.