docs(popover): add panel slot example

Resolves #763

docs/components/content/examples/PopoverExampleSlot.vue

@@ -0,0 +1,11 @@
+<template>
+  <UPopover>
+    <UButton color="white" label="Open" trailing-icon="i-heroicons-chevron-down-20-solid" />
+
+    <template #panel="{ close }">
+      <div class="p-8">
+        <UButton label="Close" @click="close" />
+      </div>
+    </template>
+  </UPopover>
+</template>