I've been moving my documentation into Github because of all the benefits that arise from having version control and its always available. That means using Markdown for formatting. There are a lot of Markdown editors around, and I've experimented with a few: Mark Text and Typora have been my favourites to date. Both are available across all platforms.
Recently I came across MindForger which is a markdown IDE. It allows you to work with markdown as a structured document, which is great generally, and really good for documentation.
If it was just an editor/viewer I wouldn't bother recommending it here. But it has some really unique features which might be appealing. It uses text analysis to generate information/knowledge from your content. It is much more than a keyword search. It's a bit like google, using powerful algorithms, combined with tags, to create suggestions and filters to locate related information across many documents.
It can perform the text analysis on any folder that you point at. So, for instance, you can download a github repository and immediately get deep into the code and start to find relationships between different documents that wouldn't be possible otherwise.
It's Free Open Source Software ( FOSS ), and downloadable for Mac, Windows, and a bunch of Linux flavours.