feat(docs): Basic mod-tap behavior docs.

docs/docs/behavior/mod-tap.md

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+---
+title: Mod-Tap
+---
+
+## Summary
+
+The Mod-Tap behavior allows varying the effect of pressing and releasing a key position depending
+on whether it is used with other simultaneous key presses at the same time.
+
+If pressed and released independently, the Mod-Tap behavior will send the press and release events
+for the configure keycode. If pressed and held while another key is pressed and released, then
+the configured modifiers will be applied to that _other_ key press, and no press will be generated
+on the release of the Mod-Tap key.
+
+## Mod-Tap
+
+The Mod-Tap behavior either acts as a held modifier, or as a tapped keycode.
+
+### Behavior Binding
+
+- Reference: `&mt`
+- Parameter #1: The modifiers to be used when activating as a modifier, e.g. `MOD_LSFT`
+- Parameter #2: The keycode to sent when used as a tap, e.g. `A`, `B`.
+
+Example:
+
+```
+&mt MOD_LSFT A
+```