refactor(ci): cache python packages for pages build

.gitlab-ci-mkdocs.yml

 
+variables:
+  PIP_CACHE_DIR: "$CI_PROJECT_DIR/.cache/pip"
+
+cache:
+  paths:
+    - .cache/pip
+    - venv/
+
 mkdocs:compile:
   stage: compile
   image: python:3.8
   tags:
     - docker
+  before_script:
+    - pip install virtualenv
+    - virtualenv venv
+    - source venv/bin/activate
   script:
     - pip install -r requirements.txt -U
     - mkdocs build --strict