docs: use absolute slug so the page resolves at /learn/get-started

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Author: matlegault

fern/products/home/pages/agent-onboarding.mdx

@@ -1,6 +1,6 @@
 ---
 title: Get started with Fern using a coding agent
-slug: get-started
+slug: /get-started
 description: Instructions a coding agent follows to set up Fern Docs, SDKs, or both for a user's API.
 ---