@euant Its great you want to contribute with documentation.
Gitbook seems to be a locked in service. Django Framework is an example of fantastic documentation for an opensource project and it uses RestructuredText and Sphinx which are both opensource and self hosted. The documentation presentation for Django is great but the real value is in the comprehensive nature of the documentation and the completeness. Documentation seems to be improving lately for Nim which is fantastic. For me presentation could be improved but there is still documentation that is terse and incomplete. I would like to learn more Nim and if I had time I could put in more effort to look through the codebase to discover how things work, but I simply don't have time. Easy to use, and complete documentation makes a project more attractive and also more credible.
