docs: add conditional slot example (#5383)

site/content/examples/15-composition/04-conditional-slots/App.svelte

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+<script>
+	import Profile from "./Profile.svelte";
+</script>
+
+<Profile>
+	<span slot="name">Bob</span>
+	<span slot="email">[email protected]</span>
+</Profile>
+
+<Profile>
+	<span slot="name">Alice</span>
+	<span slot="phone">12345678</span>
+</Profile>

site/content/examples/15-composition/04-conditional-slots/Profile.svelte

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+<style>
+	section {
+		width: 200px;
+		display: grid;
+		grid-template-columns: 1fr 1fr;
+		padding: 16px;
+		box-shadow: 2px 2px 4px #dedede;
+		border: 1px solid #888;
+		margin-bottom: 16px;
+	}
+</style>
+
+<section>
+	<div>Name</div>
+	<slot name="name" />
+	{#if $$slots.email}
+		<div>Email</div>
+		<slot name="email" />
+	{/if}
+	{#if $$slots.phone}
+		<div>Phone</div>
+		<slot name="phone" />
+	{/if}
+</section>

site/content/examples/15-composition/04-conditional-slots/meta.json

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+{
+	"title": "Conditional Slots"
+}