If you want to build the documentation locally (e.g. to test your changes), the dependencies are automatically installed as part of Zulip development environment provisioning, and you can build the documentation using:
cd docs/ make html
and then opening
your browser (you can also use e.g.
firefox docs/_build/html/index.html from the root of your Zulip checkout).
You can also usually test your changes by pushing a branch to GitHub and looking at the content on the GitHub web UI, since GitHub renders Markdown.
When editing dependencies for the Zulip documentation, you should edit
requirements/docs.txt (which is used by ReadTheDocs to build the
documentation quickly, without installing all of Zulip’s dependencies).