The OVN documentation website is based on hugo, grav, and the hugo-learn-theme and written in markdown format.
If you want to contribute I recommend reading the hugo-learn-theme documentation
You can always click the “Edit this page link” at the top right of each page, but if you want to test your changes locally before submitting you can:
Fork the ovn-org/ovn-website on github
Checkout your copy locally
$ git clone ssh://git@github.com/<your-user>/ovn-website.git
$ cd ovn-website
$ make server
Open your browser on http://localhost:1313
Edit files in src, the browser should automatically reload changes.
Eventually commit, push, and pull-request your changes. You can find a good guide about the github workflow here: contributing to a github project