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

docs/docs/behaviors/sticky-layer.md

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+---
+title: Sticky Layer Behavior
+sidebar_label: Sticky Layer
+---
+
+## Summary
+
+A sticky layer stays pressed until another key is pressed. By using a sticky layer, you don't have to hold the layer key to access a layer. Instead, tap the sticky layer key to activate the layer until the next keypress.
+
+By default, sticky layers 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: `&sl`
+- Parameter #1: The layer to activate , e.g. `1`
+
+Example:
+
+```
+&sl 1
+```
+
+### Configuration
+
+You can configure a different `release-after-ms` in your keymap:
+
+```
+&sl {
+    release-after-ms = <2000>;
+};
+
+/ {
+    keymap {
+        ...
+    }
+}
+```
+
+### Advanced usage
+
+Sticky layers behave slightly different from sticky keys. They are configured to `quick-release`. This means that the layer is released immediately when another key is pressed. "Normal" sticky keys are not `quick-release`; they are released when the next key is released. This makes it possible to combine sticky layers and sticky keys as such: tap `&sl 1`, tap `&sk LSHFT` on layer 1, tap `&kp A` on base layer to output shift+A.
+
+### Comparison to QMK
+
+In QMK, sticky layers are known as 'one shot layers'.