Create submenu for 'Call for sponsors'

[0xUnicorn]

When sponsors exist on the camp sponsor page https://bornhack.dk/bornhack-2025/sponsors/ the call for sponsors gets moved to the bottom of the page.

Create a submenu showing the Call for sponsors like we do on https://bornhack.dk/bornhack-2026/program/events/ for the https://bornhack.dk/bornhack-2026/program/call-for-participation/

When no sponsors exist yet, make a redirect from the sponsor page to the CFS submenu page