Add Copilot instructions for consistent placeholder formatting in documentation (#36152)

* Initial plan

* Add placeholder formatting guidelines to Copilot instructions

Co-authored-by: wadepickett <10985336+wadepickett@users.noreply.github.com>

* Update .github/copilot-instructions.md

Changing to more concise diff format for example and using percase example

* Update copilot-instructions.md

Fixed formatting

* Fix placeholder formatting examples in instructions

fomat fix.

* Update .github/copilot-instructions.md

Adding guardrex improvement

Co-authored-by: Luke Latham <1622880+guardrex@users.noreply.github.com>

* Update .github/copilot-instructions.md

Adding guardrex fix

Co-authored-by: Luke Latham <1622880+guardrex@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: wadepickett <10985336+wadepickett@users.noreply.github.com>
Co-authored-by: Wade Pickett <wpickett@microsoft.com>
Co-authored-by: Luke Latham <1622880+guardrex@users.noreply.github.com>

Author: 3 people

.github/copilot-instructions.md

@@ -82,6 +82,9 @@ When working on an issue:
   - [ ] Number ordered lists as "1." for every item (don't use sequential numbers)
   - [ ] Use backticks around content specifically for file names (`file.txt`), folders (`folder`), file paths (`folder/file.txt`), custom types (`myVariable`, `MyClass`), raw URLs in the text (`https://www.contoso.com`), URL segments (`/product/id/199`), file extensions (`.razor`), NuGet packages (`Microsoft.AspNetCore.SignalR.Client`), and code that should never be localized
   - [ ] For Blazor's Razor components mentioned in article text, use backticks around the name of the component (example: `Counter` component)
+  - [ ] Use placeholders with braces in the format `{PLACEHOLDER NAME}` when used in URIs, code examples, and other contexts where placeholders are used. Use uppercase letters with spaces between words for the placeholder name inside the braces.
+    - [ ] Wrong: "Launch the app and navigate to `https://localhost:<port>/openapi/v1.json` to view the generated OpenAPI document."
+    - [ ] Correct: "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."
   - [ ] For any new or updated .md file, ensure the standard frontmatter (metadata) is included as specified in [Metadata for Microsoft Learn documentation.](https://learn.microsoft.com/en-us/contribute/content/metadata)
   - [ ] For any new or updated .md file added to the repository, ensure the following frontmatter (metadata) is included:
     - [ ] Metadata `ai-usage: ai-assisted` if any AI assistance was used