Add guides for setting up applications.

[mridang]

1 Reference this https://github.com/zitadel/zitadel-nextjs/blob/5e75a25aef973d6320bc3002d62e15dea22db5aa/README.md

2. Reference this https://github.com/Nuove-Grafiche-Puddu/Sveltekit-Authjs-Zitadel/blob/main/README.md

Fixes zitadel/example-auth-nextjs#5