Add Copilot instructions for consistent placeholder formatting in documentation

[wadepickett]

Issue description

In the general copilot instructions for this repository (.github/copilot-instructions.md), please add the following rule in an appropriate location to ensure placeholder formatting consistency. This is not a code specific rule.

Rule to add:

• Use placeholders with braces format: {placeholderName} consistently throughout the document when used in a URI.
• Use uppercase for the placeholder name.

  - Launch the app and navigate to `https://localhost:<port>/openapi/v1.json` to view the generated OpenAPI document.
  + Launch the app and navigate to `https://localhost:{PORT}/openapi/v1.json` to view the generated OpenAPI document, where the `{port}` placeholder is the port.

Location:
This rule should be included in the copilot instructions at:
https://github.com/dotnet/AspNetCore.Docs/blob/main/.github/copilot-instructions.md

Assigning to @wadepickett.

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Associated WorkItem - 491877

[guardrex]

BTW @wadepickett ... The Blazor node uses uppercase placeholder names (with spaces). The main doc set is all over the place with multiple formats in use.

For example, this would be {PORT} in Blazor articles. The reason that I went lowercase here was because the convention was abandoned by doc authors years ago in the main doc set and the rest of this article used either Pascal or lower camel case. Safia may have done that to match API or for some other reason.

I recommend that one standard be adopted for all articles. I can change the Blazor node usages if the team goes another way than all uppercase (with spaces).

Also note that you're telling Copilot "when used in a URI," but the placeholders are used for many more purposes. Copilot might be smart enough to use placeholders generally. If so, Copilot might be able to adopt the rule without scoping it to URIs.

[wadepickett]

@guardrex thanks. All good points and helpful!

Upper case: I'll correct. You are right, I was using that latest example correction as the example here.

Scope of issue addressed here:

• I agree, placeholders are used for many other purposes and therefore there is a much larger scope to consider. For now, I am just having it correct in URI's with that narrow scope.
• We are on the same page I think that within that narrow scope, it has been a problem throughout the repo for URI's, which is why I am adding the rule for the narrow scope now.
• Eventually a broader scope of rules for handling proper place holders in C#, CLI, JavaScript, HTML, etc, would be very nice to address. I don't feel I could trust it to consistently handle it across all scopes with a very generalized rule.