add anchor links to headings (#303)

# Description
- feat(headings): add anchor links to headings

## Screenshots
| before | after |
|--------|-------|
|
![image](https://user-images.githubusercontent.com/2574275/219791007-b14d54a0-4c2f-4432-90c5-fd586df4c76e.png)
|
![image](https://user-images.githubusercontent.com/2574275/219790961-9b1daaf4-99fc-470c-a20b-0116cfb8bfdc.png)
|

astro.config.mjs

@@ -6,6 +6,24 @@ import image from "@astrojs/image";
 import mdx from "@astrojs/mdx";
 
 import sitemap from "@astrojs/sitemap";
+import rehypeAutolinkHeadings from "rehype-autolink-headings";
+import { h } from 'hastscript';
+
+const AnchorLinkIcon = h(
+	'svg',
+	{
+		width: 16,
+		height: 16,
+		version: 1.1,
+		viewBox: '0 0 16 16',
+		xlmns: 'http://www.w3.org/2000/svg',
+	},
+	h('path', {
+		fillRule: 'evenodd',
+		fill: 'currentcolor',
+		d: 'M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z',
+	})
+);
 
 // https://astro.build/config
 export default defineConfig({
@@ -23,5 +41,29 @@ export default defineConfig({
     shikiConfig: {
       theme: 'dracula-soft',
     },
+    rehypePlugins: [
+      [
+        rehypeAutolinkHeadings,
+          {
+            properties: {
+              class: 'anchorLink',
+            },
+            behavior: 'before',
+            group: ({ tagName }) =>
+              h(`div.headingWrapper.level-${tagName}.flex.flex-row.items-baseline.prose-a:mr-4`, {
+                tabIndex: -1,
+              }),
+            content: () => [
+              h(
+                `span.anchor-icon.align-baseline`,
+                {
+                  ariaHidden: 'true',
+                },
+                AnchorLinkIcon
+              ),
+            ],
+          },
+      ],
+    ],
   },
 });