ops: list mkdocs as doc requirement

Author: jannismain

pyproject.toml

@@ -35,6 +35,7 @@ fallback_version = "0.0.0"
 dev = ["black", "pre-commit", "ruff"]
 test = ["pytest", "pytest-venv", "pytest-xdist", "pexpect"]
 doc = [
+    "mkdocs",
     "mkdocs-material",
     "mkdocs-git-revision-date-localized-plugin",
     "mkdocs-literate-nav",