cylc.org: revert url change

* cylc.org does not redirect properly to cylc.github.ui causing the
  URL to be trimmed.

setup.cfg

@@ -23,7 +23,7 @@ description = A workflow engine for cycling systems
 long_description=file: README.md
 long_description_content_type=text/markdown
 project_urls =
-    Documentation = https://cylc.org/cylc-doc
+    Documentation = https://cylc.github.io/cylc-doc/latest/html/index.html
     Source = https://github.com/cylc/cylc-flow
     Tracker = https://github.com/cylc/cylc-flow/issues
 keywords =