My favorite for now stays mkdocs (http://www.mkdocs.org/). For relative small ones with the material theme (http://squidfunk.github.io/mkdocs-material/) and bigger ones (because of top bar) with bootswatch theme (http://mkdocs.github.io/mkdocs-bootswatch/). It works the same, just builds a site from your MD files but creates a static site which can be hosted anywhere (e.g. github/gitlab pages via auto CI). Editing and creating your own theme is also easy and documented.

Installation is easiest I found for docs sites (pip install mkdocs) and it's blazingly fast. Comes with good themes and proper responsive design (saw some issues in Raneto on my ios). Has proper live reloading and build in serve functionality.

For large projects (enterprise size) we have made our own version based on what I learned from the Polymer docs (https://github.com/Polymer/doc) but removed the GAE requirement. Also, we are considering if using SO Enterprise is a good solution. Personally I'm in favor for in house docs and knowledge sharing for that. It's so flexible and people are already used to it. But that is a different ball park considering the OP.

To end with a positive note. Raneto seems to support much of the same as mkdocs or any of its alternatives. It looks good and whilst static site is something I want I understand it is not a deal breaker for all. In addition, Raneto mentions authentication is built in which is a nice thing if I want to host for internal stuff on a public domain so external customers can also use it without requiring a VPN account. Overall, nice work! Not for today, but I have bookmarked it.