Refactor sticky key and sticky layer doc pages to mention the underlying sticky key behavior

[caksoylar]

Both sticky keys (i.e. &sk) and sticky layers &sl are instances of the underlying sticky key behavior https://zmk.dev/docs/config/behaviors#sticky-key with different bindings values. However this isn't documented well under the behavior pages.

Ideally, "sticky key" page could describe both the specific &sk behavior and the generic zmk,behavior-sticky-key behavior underlying it. Mod-morph page is an example that does this with &gresc and zmk,behavior-mod-morph. "Sticky layer" page should also have a remark saying it is a sticky key under the hood with bindings = <&mo>;, similar to info box present in https://zmk.dev/docs/behaviors/mod-tap.