docs(linux): Keyman for Linux command line should be documented in "Getting Started"

https://help.keyman.com/products/linux/11.0/ currently only contains very basic 'install keyboard' GUI documentation. This is not sufficient; the command line tools should also be documented (the man pages, plus an overview). See https://manpages.debian.org/unstable/keyman/ for sample content (which should of course be generated from the source man pages at release time).