App Model topic+sample update

[guardrex]

Fixes #21971
Addresses #5495

Internal Review Topic

Thanks @babula38! 🚀 ... We'll see if Rick is ok with a single 3.1 sample because the text is version agnostic AFAICT. I ran the upgraded app locally, and its demos seem to perform ok.

Rick, I didn't perform a full line-by-line edit due to time constraints 🏃😅. I just touched up the future tense and dropped a few stray spaces at the ends of paragraphs.

UPDATE: ... but I dropped in the API doc cross-links and fixed other nits and whatnots ... did some loc work along the way. There are a couple of API things that don't seem to exist any longer. I don't mean the internal things that don't show up in API docs. I'm referring to ...

• ActionContext.ActionDescriptor.Properties ... not sure where that is or if it went away at some point ... or it was never right. Cross-reference: https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.mvc.actioncontext.actiondescriptor
• I can't find the API for MustBeInRouteParameterModelConvention, but it works in the 3.1 sample app.
• Can't find WebApiApplicationModelConvention.

Finally, I updated the headings. I elevated the H3s under conventions to surface them in the sidebar ToC. I think that makes it a little easier to find a specific convention scenario.

Again tho, I didn't perform a complete UE pass. On Monday morning, I'll hit it one last time. May as well ... I went this far with it. 😁

[Rick-Anderson]

Wow, nice work. You even cleaned up the writing.

[guardrex]

Cat got me up at 3:30am❗ Rotten 😼❗ Nothing else to do at that hour.

[guardrex]

I'm going to read and sweep this one last time on Monday morning. I doubt anyone will get back to it for a long time, so it's probably worth a last look before merging.

For example, I need to do something about ...

• ActionContext.ActionDescriptor.Properties ... not sure where that is or if it went away at some point ... or it was never right. It might mean to say ActionContext.ActionDescriptor properties ("properties" not code-fenced). Cross-reference: https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.mvc.actioncontext.actiondescriptor Resolved.
• Can't find WebApiApplicationModelConvention. I need to go an API hunt. Won't bother. The shim was dropped at 3.0, so I'll version this section, and it will drop when 2.x drops from the docs.