Configure SignalR hub URL endpoint

[guardrex]

Fixes #36119

Brennan, Wade ...

• From the issue, it seems that the main problem was just the hub endpoint location config in that scenario. Therefore, I'm angling this addition to the SignalR config article to discuss how to set the URL in code versus statically setting it. I noticed that there isn't an H3 URL configuration section, so that's the first draft on how to add this coverage (and I added it first among the existing H3 sections because it seems to be the most basic among them). If you want to get into more detail on some specific proxy/LB config, then I'll need to know what to cover (and possibly where, since this might move).
• I use "URL" as opposed to "URI" because the API is named "WithUrl" ... "to use HTTP-based transports to connect to the specified URL and transports."
• I cross-link from the proxy/LB and Blazor SignalR articles.
• Removed UseDefaultCredentials because it's covered elsewhere in the article and not necessarily germane to the scenario (let me know if I'm incorrect about that, and I'll add it back).
• OMG! ... added it to all of the INCLUDES files so that it can be cross-linked from other articles. There's massive content duplication across seven additional files with broken bookmarks in every prior release version of the article. Is the intention to keep going with this to .NET 15 ... .NET 20 (!!!) ... and beyond? I recommend MS stop using this approach for versioning content.

[wadepickett]

At this point it is already an absolute URI string I think. Do we need to use Navigation.ToAbsoluteUri() again? Maybe should be just .WithUrl(uri)?