docs(sticky keys): document sticky keys and sticky layers

docs/docs/behaviors/sticky-key.md

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+---
+title: Sticky Key Behavior
+sidebar_label: Sticky Key
+---
+
+## Summary
+
+A sticky key stays pressed until another key is pressed. It is often used for 'sticky shift'. By using a sticky shift, you don't have to hold the shift key to write a capital.
+
+By default, sticky keys stay pressed for a second if you don't press any other key. You can configure this with the `release-after-ms` setting (see below).
+
+### Behavior Binding
+
+- Reference: `&sk`
+- Parameter #1: The keycode , e.g. `LSHFT`
+
+Example:
+
+```
+&sk LSHFT
+```
+
+You can use any keycode that works for `&kp` as parameter to `&sk`:
+
+```
+&sk LG(LS(LA(LCTRL)))
+```
+
+### Configuration
+
+You can configure a different `release-after-ms` in your keymap:
+
+```
+&sk {
+    release-after-ms = <2000>;
+};
+
+/ {
+    keymap {
+        ...
+    }
+}
+```
+
+### Advanced usage
+
+Sticky keys can be combined; if you tap `&sk LCTRL` and then `&sk LSHFT` and then `&kp A`, the output will be ctrl+shift+a.
+
+### Comparison to QMK
+
+In QMK, sticky keys are known as 'one shot mods'.