docs: Fix help.github.com link check

For some reason, help.github.com will only serve compressed content, and
will reply with a 403 error without any "Accept-encoding" headers.
This causes 'make docs_check_links' to report help.github.com links as being
dead.
This is tracked in github/docs#17358

For this fix to work, quay.io/crcont/docs-builder:latest must be rebuilt
and pushed to quay.
This can be rebuilt and tested locally with:

$ (cd images/docs-builder && podman build -t quay.io/crcont/docs-builder:latest .)
$ make docs_check_links

images/docs-builder/asciidoc-link-check-config.json

@@ -4,5 +4,13 @@
         { "pattern": "^http://api.crc.testing" },
         { "pattern": "https://console-openshift-console.apps-crc.testing" },
         { "pattern": "http://proxy.example.com" }
+    ],
+    "httpHeaders": [
+        {
+          "urls": ["https://help.github.com"],
+          "headers": {
+            "Accept-Encoding": "gzip, deflate"
+          }
+        }
     ]
 }