Rondel is a text editor. Nothing special about it, you've used text editors before. But Rondel allows you to insert Markdown formatting codes, either by hand or from its menus, and to generate HTML files from its own text files.
Installation: Move (don't copy) the .hpkg file into /boot/home/config/packages on any Haiku installation with Package Management enabled. If your Haiku installation is quite up-to-date, you should be able to double-click the .hpkg and have it load into HaikuDepot...