I couldn't find any reference to *how* the keymap format actually needs to look
like if you want multiple keysyms per level. So let's add a test for it and a
minimal documentation entry.
- Add table of contents
- Add terminology section
- (WIP) Add Introduction to the format
- Improve the keycode section
- Improve the interpret section
- Add guide to create and use modifiers
- (WIP) Add actions documentation
- Add cross-references
- Add keysyms header to documentation