Improve OnNotFound example

[guardrex]

Fixes #35468
Addresses #34437

Ilona ... IMO, much better than what I had here. Your example concept is GrEaT! 🎉 A few notes ...

• This adopts my strategy of using fully working, cut-'n-paste examples for demos, which devs enjoy. Eventually, this example code will make its way into the Blazor sample apps.
• I changed some bits here and there, such as making the Context into a Heading property, and I'm 👂 for improvements.
• The Routing article is now too long and too full of advanced concepts, so I'll be creating a Routing node of articles or a single article in the reference docs and downsize this "fundamentals" article to focus on basic concepts. For now, we'll just place this new content here, but I think this will become an advanced scenario for a different article later. That work is tracked by Split out advanced routing and nagivation coverage (dotnet/AspNetCore.Docs #35474).

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Internal previews

📄 File	🔗 Preview link
aspnetcore/blazor/fundamentals/routing.md	aspnetcore/blazor/fundamentals/routing

[ilonatommy]

Thank you for putting it on paper so neatly. Looks great