[docs] add <svelte:element> example (#7439)

Author: oekazuma

site/content/examples/17-special-elements/02-svelte-element/App.svelte

@@ -0,0 +1,12 @@
+<script>
+	const options = ['h1', 'h3', 'p'];
+	let selected = options[0];
+</script>
+
+<select bind:value={selected}>
+	{#each options as option}
+		<option value={option}>{option}</option>
+	{/each}
+</select>
+
+<svelte:element this={selected}>I'm a {selected} tag</svelte:element>

site/content/examples/17-special-elements/02-svelte-element/meta.json

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