test: Ignore CERN Twiki URLs during Sphinx linkcheck (#2276)

* Ignore URLs for CERN's Twiki instance https://twiki.cern.ch/ from
  Sphinx linkcheck tests as CERN seems to have problems regularly
  maintaining their SSL certs which can lead to SSL errors.

  ```
  Caused by SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED]
  certificate verify failed: unable to get local issuer certificate (_ssl.c:1002)'))
  ```

docs/conf.py

@@ -519,6 +519,8 @@ def setup(app):
     r'https://indico.desy.de/event/22731/.*',
     # https://indico.belle2.org/event/8470/contributions/55871/ is frequently generating 403 Client Error
     r'https://indico.belle2.org/event/8470/.*',
+    # CERN doesn't maintain its SSL certs well enough to not have SSLErrors
+    r'https://twiki.cern.ch/.*',
     # tags for a release won't exist until it is made, but the release notes
     # and ReadTheDocs need to reference them
     r'https://github.com/scikit-hep/pyhf/releases/tag/.*',