ci: Export HTML documentation as artifacts

We build the HTML documentation. For feature branches, it is convenient
to be able to view the documentation easily. Expose them as artifacts
and add a link to the browser underneath the pipeline in the MR.

Unfortunately, it does not seem to be possible to link directly to the
HTML.

.gitlab-ci.yml

@@ -48,6 +48,11 @@ build:
   <<: *build_one_driver
   <<: *build
 #  <<: *check_abi
+  artifacts:
+    expose_as: "HTML Documentation"
+    paths:
+    - _build/doc/html/
+    expire_in: 1 week
 
 test:
   stage: test