Update copilot-instructions.md

Co-authored-by: Luke Latham <[email protected]>

Author: wadepickett

.github/copilot-instructions.md

@@ -51,7 +51,8 @@ This document contains repository-specific instructions for GitHub Copilot when
 
 ## Links and References
 
-- Use **relative links** for files within this repo
+- Use relative links for files within this repo
+- When adding external links into a document, strip out the language/culture segment of the URL if present (example: remove the `en-us` or `en` part of links to English versions of webpages)
 - For learn.microsoft.com links, **remove** `https://learn.microsoft.com/en-us` from URLs
 - Use cross-references for APIs: `<xref:api-doc-ID>`
 - Get doc ID from the relevant XML doc in dotnet-api-docs