Fix CI: Ignore LinkedIn links in markdown-link-check (#1955)

## Summary

LinkedIn returns 999 status codes for automated link checkers, causing
false positives in CI markdown link checking. This is well-documented
LinkedIn anti-bot behavior.

## Change

Added LinkedIn to the  in 

## Impact

- Fixes failing markdown-link-check CI workflow
- Prevents future false positives from LinkedIn URLs in documentation

## Related

This fixes the CI failure seen in docs/testimonials/hvmn.md and any
other files with LinkedIn profile links.

Author: justin808

.github/markdown-link-check-config.json

@@ -26,6 +26,9 @@
     },
     {
       "pattern": "^https://reactrails\\.slack\\.com"
+    },
+    {
+      "pattern": "^https://www\\.linkedin\\.com"
     }
   ],
   "replacementPatterns": [