feat: add robots and sitemap generation, enhance documentation page with copy markdown button

- Introduced `robots.ts` for defining robots.txt rules and sitemap URL.
- Added `sitemap.ts` to generate a sitemap based on available documentation pages.
- Enhanced the documentation page to include a "Copy as Markdown" button for easy copying of content.
- Created `copy-markdown-button.tsx` component for clipboard functionality.
- Implemented `llms.txt` route to provide a text-based overview of documentation links.

apps/docs/components/copy-markdown-button.tsx

@@ -0,0 +1,30 @@
+"use client";
+
+import { useCopyButton } from "fumadocs-ui/utils/use-copy-button";
+import { Check, Copy } from "lucide-react";
+
+export function CopyMarkdownButton({ markdown }: { markdown: string }) {
+	const [checked, onClick] = useCopyButton(() => {
+		navigator.clipboard.writeText(markdown);
+	});
+
+	return (
+		<button
+			type="button"
+			className="inline-flex items-center gap-1.5 rounded-md border bg-fd-secondary px-3 py-1.5 text-xs font-medium text-fd-secondary-foreground transition-colors hover:bg-fd-accent hover:text-fd-accent-foreground"
+			onClick={onClick}
+		>
+			{checked ? (
+				<>
+					<Check className="size-3.5" />
+					Copied!
+				</>
+			) : (
+				<>
+					<Copy className="size-3.5" />
+					Copy as Markdown
+				</>
+			)}
+		</button>
+	);
+}