Don't update the active section for headings below level 3

Fixes #2365

site/src/routes/docs/_sections.js

@@ -125,7 +125,7 @@ export default function() {
 
 					return `
 						<h${level}>
-							<span id="${slug}" class="offset-anchor"></span>
+							<span id="${slug}" class="offset-anchor" ${level === 4 ? 'data-level=4' : ''}></span>
 							<a href="docs#${slug}" class="anchor" aria-hidden="true"></a>
 							${text}
 						</h${level}>`;

site/src/routes/docs/index.svelte

@@ -18,7 +18,9 @@
 	let show_contents = false;
 
 	onMount(() => {
-		const anchors = container.querySelectorAll('[id]');
+		// don't update `active_section` for headings below level 3, see _sections.js
+		const anchors = container.querySelectorAll('[id]:not([data-level="4"])');
+
 		let positions;
 
 		const onresize = () => {
@@ -372,4 +374,4 @@
 	<button on:click="{() => show_contents = !show_contents}">
 		<Icon name="{show_contents? 'close' : 'menu'}"/>
 	</button>
-</aside>
+</aside>