This is the source code for the official i3 website: https://i3wm.org/.
You will need Jekyll and AsciiDoc.
First, generate the docs from the _docs folder with make -C _docs -j.
Move the HTML files from _docs to docs (notice the underscore!): mv _docs/*.html docs/.
Then, use jekyll build and you can finally serve the website locally with jekyll serve.
Changes to the docs should happen first on the i3 source code repository, in the docs folder. So, for example, if you want to make changes to the userguide you will have to submit a pull request first to the i3 github repository and then (optionally, if your change should be applied immediately to the userguide) submit a PR in this repository as well.
Docs use the AsciiDoc format.
Edit the file in _docs, then follow the building process above.
When submitting a pull request, include both the changed files in _docs and
the corresponding generated HTML files.