Ignore SSLErrors

This avoids being someone else's uptime monitor and makes linkchecker
failures actionable from our side.

This is a minor change since:

- It does not affect the content or presentation of the website.
- It can be seen as a fine-tuning of the QA implementation, but without
  changing the QA process per-se.

linkchecker.conf

@@ -27,6 +27,7 @@ ignoreerrors=
     # Avoid being someone else's uptime monitor :)
     ^https:// ^ReadTimeout
     ^https:// ^ConnectionError
+    ^https:// ^SSLError
 
 [AnchorCheck]