docs: add stub documentation for router composables

docs/content/3.api/1.composables/on-before-route-leave.md

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+---
+title: "onBeforeRouteLeave"
+description: The onBeforeRouteLeave composable allows registering a route guard within a component.
+---
+
+# `onBeforeRouteLeave`
+
+The `onBeforeRouteLeave` composable adds a navigation guard that triggers whenever the component for the current location is about to be left.
+
+::ReadMore{link="https://router.vuejs.org/api/index.html#onbeforerouteleave"}
+::

docs/content/3.api/1.composables/on-before-route-update.md

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+---
+title: "onBeforeRouteUpdate"
+description: The onBeforeRouteUpdate composable allows registering a route guard within a component.
+---
+
+# `onBeforeRouteUpdate`
+
+The `onBeforeRouteUpdate` composable adds a navigation guard that triggers whenever the component for the current location is about to be updated.
+
+::ReadMore{link="https://router.vuejs.org/api/index.html#onbeforerouteupdate"}
+::