Add LLM text retrieval route

### Summary of Changes
- Introduced a new `GET` function in `route.ts` to aggregate and return LLM texts from both local and API sources.
- Set `revalidate` to false to optimize static content delivery.
- Implemented response formatting to return plain text with appropriate headers.

This addition enhances the LLM functionality by consolidating text retrieval into a single endpoint, improving efficiency and usability.

docs/src/app/llms-full.txt/route.ts

@@ -0,0 +1,15 @@
+import { getLLMText } from '../../../lib/get-llm-text';
+import { apiSource, source } from '../../../lib/source';
+
+export const revalidate = false;
+
+export async function GET() {
+  const pages = [...source.getPages(), ...apiSource.getPages()];
+  const texts = await Promise.all(pages.map(getLLMText));
+
+  return new Response(texts.join('\n\n'), {
+    headers: {
+      'Content-Type': 'text/plain; charset=utf-8',
+    },
+  });
+}