feat(website): add anchor links to docs headings

Hover a heading to reveal a # link on the left that updates the URL
hash without layout shift, matching GitHub README behavior.

packages/website/src/routes/docs/$.tsx

@@ -1,6 +1,7 @@
 import { createFileRoute } from "@tanstack/react-router";
 import ReactMarkdown from "react-markdown";
 import { getDoc } from "~/docs";
+import { docsRehypePlugins } from "~/docs-rehype";
 import { pageMeta } from "~/meta";
 
 export const Route = createFileRoute("/docs/$")({
@@ -24,5 +25,5 @@ function DocsPage() {
     return <p className="text-muted-foreground">Doc not found.</p>;
   }
 
-  return <ReactMarkdown>{doc.content}</ReactMarkdown>;
+  return <ReactMarkdown rehypePlugins={docsRehypePlugins}>{doc.content}</ReactMarkdown>;
 }

packages/website/src/routes/docs/index.tsx

@@ -1,6 +1,7 @@
 import { createFileRoute } from "@tanstack/react-router";
 import ReactMarkdown from "react-markdown";
 import { getDoc } from "~/docs";
+import { docsRehypePlugins } from "~/docs-rehype";
 import { pageMeta } from "~/meta";
 
 export const Route = createFileRoute("/docs/")({
@@ -17,5 +18,5 @@ export const Route = createFileRoute("/docs/")({
 function DocsIndex() {
   const doc = getDoc("");
   if (!doc) return <p className="text-muted-foreground">Doc not found.</p>;
-  return <ReactMarkdown>{doc.content}</ReactMarkdown>;
+  return <ReactMarkdown rehypePlugins={docsRehypePlugins}>{doc.content}</ReactMarkdown>;
 }