docs: add `<svelte:document>` example (#8387)

site/content/examples/17-special-elements/05-svelte-document/App.svelte

@@ -0,0 +1,10 @@
+<script>
+	let selection = '';
+
+	const handleSelectionChange = (e) => selection = document.getSelection();
+</script>
+
+<svelte:document on:selectionchange={handleSelectionChange} />
+
+<p>Select this text to fire events</p>
+<p>Selection: {selection}</p>

site/content/examples/17-special-elements/05-svelte-document/meta.json

@@ -0,0 +1,3 @@
+{
+	"title": "<svelte:document>"
+}