feat(config): create anchor links for headings (#74)

This makes it easier to e.g. reference particular atuin config keys in
the documentation.

Preview:

![Screenshot 2024-08-18 at 12-19-40 Syncing dotfiles Atuin
Docs](https://github.com/user-attachments/assets/d31d85cd-d33e-4140-a46c-e9acd875bef3)

astro.config.mjs

@@ -1,10 +1,29 @@
 import { defineConfig } from 'astro/config';
 import starlightLinksValidator from 'starlight-links-validator'
 import starlight from '@astrojs/starlight';
+import rehypeSlug from 'rehype-slug';
+import rehypeAutolinkHeadings from 'rehype-autolink-headings';
 
 // https://astro.build/config
 export default defineConfig({
   site: "https://docs.atuin.sh",
+  markdown: {
+    rehypePlugins: [
+      rehypeSlug,
+      [
+        rehypeAutolinkHeadings,
+        {
+          behavior: 'wrap',
+          headingProperties: {
+            className: ['anchor'],
+          },
+          properties: {
+            className: ['anchor-link'],
+          },
+        }
+      ]
+    ]
+  },
   integrations: [
     starlight({
       plugins: [starlightLinksValidator()],

package.json

@@ -13,6 +13,8 @@
     "@astrojs/check": "^0.5.6",
     "@astrojs/starlight": "^0.20.1",
     "astro": "^4.4.6",
+    "rehype-autolink-headings": "^7.1.0",
+    "rehype-slug": "^6.0.0",
     "sharp": "^0.33.2",
     "starlight-links-validator": "^0.5.3",
     "typescript": "^5.3.3"