It is written in Markdown and the source is hosted on this GitHub repository: openfisca/openfisca-doc.
Every reader is encouraged to participate in the improvement of the documentation.
On each page there is a link named “Edit this page”. Just click on it and you’ll jump on GitHub on the Markdown source file of the page. Then edit the file as explained on this GitHub documentation page: editing-files-in-another-user-s-repository.
Then save the file and create a pull request which will be accepted if relevant.
Check this documentation’s README.