Deploy docs only on pushes to develop

As discussed in the telecon

.gitlab-ci.yml

@@ -246,7 +246,7 @@ docs/pdf:
 # Deploy stage #
 ################
 
-# Publish the documentation only for changes to master, and only when all tests pass.
+# Publish the documentation only for changes to develop, and only when all tests pass.
 #
 # Useful reading:
 #   https://git.ligo.org/lscsoft/example-ci-project/wikis/automatically-produce-documentation
@@ -257,7 +257,7 @@ pages:
     - job: docs/html
   only:
     refs:
-      - master
+      - develop
   script:
     - mv docs/build/html public
   artifacts: