Add retry logic for transient HTTP errors in markdown link checker

Configure markdown-link-check to retry on transient server errors (500, 502, 503, 504)
in addition to rate limit errors (429). This prevents CI failures from flaky external
links that temporarily return 502 Bad Gateway or similar intermittent errors.

Changes:
- Increase retryCount from 3 to 5 for better resilience
- Add explicit retry configuration with 10-second delays between attempts
- Configure retries for HTTP status codes: 429, 500, 502, 503, 504

This should resolve the flaky test failures where external links like
https://github.com/shakacode/react_on_rails_demo_ssr_hmr occasionally return 502 errors.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

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

@@ -41,7 +41,12 @@
   ],
   "timeout": "20s",
   "retryOn429": true,
-  "retryCount": 3,
+  "retryCount": 5,
   "fallbackRetryDelay": "30s",
-  "aliveStatusCodes": [200, 206]
+  "aliveStatusCodes": [200, 206],
+  "retry": {
+    "maxRetries": 5,
+    "retryAfterMs": 10000,
+    "retryOn": [429, 500, 502, 503, 504]
+  }
 }