Move dependencies to requirements.txt

.gitlab-ci.yml

 image: python:3.8-buster
 
 before_script:
-  - pip install mkdocs
-  # Add your custom theme if not inside a theme_dir
-  # (https://github.com/mkdocs/mkdocs/wiki/MkDocs-Themes)
-  # - pip install mkdocs-material
+  - pip install -r requirements.txt
 
 test:
   stage: test

requirements.txt

+# Documentation static site generator & deployment tool
+mkdocs>=1.1.2
+
+# Add your custom theme if not inside a theme_dir
+# (https://github.com/mkdocs/mkdocs/wiki/MkDocs-Themes)
+#  mkdocs-material>=5.4.0