docs: Document exception inheritance and sealed exception hierarchies

[marcelomendoncasoares]

Summary

Exception models support extends and sealed, which is what makes an exhaustive switch over server failures possible in the app. The error handling page only covered flat exceptions, and the model reference listed both keywords as class-only.

Changes

• Add an "Exception hierarchies" section to the error handling page.
• Show extends and sealed on exception model definitions.
• Add an endpoint that throws the concrete child exceptions and an app that catches the base type and switches over the subtypes exhaustively.
• Explain what a sealed base buys the app when a new exception is added later.
• Cover hierarchies in shared packages, including the rule that every subtype has to stay in the package declaring the base.
• State the validation rule that exceptions and regular models cannot be mixed in one hierarchy.
• Mark sealed as applicable to exceptions in the model reference, adjust the extends note, and link the error handling page from Related.

Reference

• feat: Support extends and sealed properties on Exception models serverpod#5307
• Support extends and sealed properties on Exception models serverpod#4249

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