docs: document security annotations required for custom error views

[Artur-]

Custom error handler views like RouteNotFoundError subclasses need security annotations (e.g., @AnonymousAllowed) when using VaadinSecurityConfigurer, which denies all unannotated views by default. Without this, users see a generic error page instead of their custom not-found page.

Adds cross-references between the routing exceptions, protect views, security configurer, and enabling security pages so users can discover this requirement from any entry point.

Fixes #5378

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Built from 04a3d31

[mcollovati]

Adding the security annotation is not enough to make the error view work. By default, both Spring Security and Vaadin have a deny-all policy for all request paths that are not Vaadin routes or are explicitly configured with a request matcher.
This means that for a non-existing page, Spring Security blocks access before Vaadin servlet is completely reached.
To make the custom not found page, in addition to the security annotations, the anyRequest matcher should be relaxed by using VaadinSecurityConfigurer.anyRequest(..) (or passing null and handle it directly with Spring Security).