Configure link check to use Accept-Encoding header for docs.github.com links

The link check job of the "Check Markdown" workflow was subject to frequent intermittent spurious failures in other
repositories, caused by links under the docs.github.com domain returning 403 HTTP status.

Others experiencing the same problem reported that they were able to work around the problem by providing a custom
`Accept-Encoding` HTTP request header. Although I was not able to find any explanation of why, it does resolve the
problem.

Even if this specific project does not contain any links to the problematic domain, this is a standardized configuration
file maintained in a centralized collection of reusable assets which are intended to be applicable to any Arduino
tooling project. So the copies of these assets should be kept in sync with the upstream files, even in cases where the
sync does not provide any immediate benefit to the project.

Author: per1234

.markdown-link-check.json

@@ -1,4 +1,12 @@
 {
+  "httpHeaders": [
+    {
+      "urls": ["https://docs.github.com/"],
+      "headers": {
+        "Accept-Encoding": "gzip, deflate, br"
+      }
+    }
+  ],
   "retryOn429": true,
   "retryCount": 3,
   "aliveStatusCodes": [200, 206]