docs(group): explain how group catch-all NotFound routes replace RouteNotFound handlers

[vishr]

When a group has middlewares, Group#Use() registers catch-all RouteNotFound routes for the group prefix and prefix + /* so that the middlewares also run for unmatched paths. A custom NotFound handler registered for those paths before that call is replaced. This is by design (#2411, TestGroup_RouteNotFoundWithMiddleware), but the Group.RouteNotFound doc did not say so.

This documentation-only change adds a note to Group.RouteNotFound that states the rule:

• The catch-all routes: each Group#Use() call that leaves the group with middlewares (re)registers them with the group middlewares, unless the Echo-wide Config.NoGroupAutoRegister404Routes is set. Echo#Group() and Group#Group() make this call when the new group has own or inherited middlewares.
• Which route wins: for the same route path the last registered NotFound route wins, and it runs only the group middlewares it was registered with. The catch-alls replace handlers registered earlier for those paths by any group or by Echo, and a handler registered later replaces them.
• Practical advice: register custom handlers on the group after the last such call for its prefix to keep its middlewares.
• Non-overwriting routers: on a Router that does not allow overwriting routes, that registration panics. Use the router-wide RouterConfig.NotFoundHandler (see TestGroup_RouteNotFoundUsesRouterConfig), or set Config.NoGroupAutoRegister404Routes and register both paths yourself on the group whose middlewares should run for them.

Every statement in the note was checked with throwaway tests against the default, non-overwriting, NotFoundHandler, and ConcurrentRouter router configurations.

Refs #3153