docs: added an entry on input bindings

This explains what pressing different keys / pressing certain buttons does.

Author: Tygo-van-den-Hurk

docs/input-bindings.md

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+# Input Bindings
+
+Slyde uses keybindings, and mouse events to make your life easier. I tried to stick to common keybindings if similar software.
+
+## Next Slide
+
+Press one of the following keys to go to the next slide: `Enter`, `Space`, `d`, `l`, and `ArrowRight`. As well as pressing the primary mouse button, or scrolling down.
+
+## Previous Slide
+
+Press one of the following keys to go to the previous slide: `a`, `h`, and `ArrowLeft`. As well as pressing the secondary mouse button, or scrolling up.
+
+## Full Screen
+
+Press `F11` or `f` to toggle between full screen mode and a normal browser window.