ci: install funkwhale_api package for docs building

The funkwhale_api version is guessed from the package, so the package needs to be installed.

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2332>

.gitlab-ci.yml

@@ -322,6 +322,7 @@ build_docs:
     - apt-get update
     - apt-get install -y graphviz
     - poetry install
+    - poetry run pip install --no-deps --editable ../api
     - git branch stable --track origin/stable || true
     - git branch develop --track origin/develop || true
   script: