[docs] add `<svelte:element>` example

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

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+<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

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+{
+	"title": "<svelte:element>"
+}