docs: add Lucia for authentication (#63339)

I used Lucia in my last App Router project and it was a super smooth
experience. I would have put it on top of the list, because it is open
source, you keep your user data, it is not a paid service, and has no
lock-in, however, I though I would have to put it below Clerk, because
Clerk is official Vercel partner (?). Anyway, would love to see Lucia in
the list!

Co-authored-by: Sam Ko <sam@vercel.com>

docs/02-app/index.mdx

@@ -37,6 +37,7 @@ Here are some common authentication solutions that support the App Router:
 
 - [NextAuth.js](https://next-auth.js.org/configuration/nextjs#in-app-router)
 - [Clerk](https://clerk.com/docs/quickstarts/nextjs)
+- [Lucia](https://lucia-auth.com/getting-started/nextjs-app)
 - [Auth0](https://github.com/auth0/nextjs-auth0#app-router)
 - [Stytch](https://stytch.com/docs/example-apps/frontend/nextjs)
 - [Kinde](https://kinde.com/docs/developer-tools/nextjs-sdk/)