fix: use single source of deps

Closes coop-cloud/organising#564

Dockerfile

@@ -8,7 +8,4 @@ WORKDIR /docs
 
 RUN apk add --no-cache curl
 
-RUN pip install \
+RUN pip install -r requirements.txt
-  mkdocs-material~=9.5.7 \
-  mkdocs-material-extensions~=1.3.1 \
-  mkdocs-awesome-pages-plugin==2.9.2