I highly recommend AsciiDoc (specifically Asciidoctor)! It has a ton of great features, has standardized syntax (unlike Markdown), can easily include sections of source code from other files (super cool and useful for KDE code documentation), can include other AsciiDoc files to make document management easy, requires no special XML formatting application, and if you follow their recommendations (keeping all sentences on one line each, using ATX sections, etc.) it makes source control via Git a breeze. And you can export to any format, including DocBook.