Create documentation artifacts

I wasn't aware our current GitLab CI config did not publish the built documentation at all. This fixes this in a way that does not exhibit the problems I was facing last time where directing GitLab to work directly in the "result/" symlink dir (the nix store) would lead to all sorts of problems.

.gitlab-ci.yml

@@ -12,7 +12,8 @@ docs:
     - "cp --recursive --no-preserve=mode result/docs/. docs/build/"
   artifacts:
     paths:
-      - "docs/build/"
+      - "docs/build/index.html"
+    expose_as: "documentation"
 
 unit-tests:
   stage: "test"