add documentation on how to add firebase env vars

[MarconLP]

these vars need documentation

normal firebase vars

# firebase
NEXT_PUBLIC_FIREBASE_API_KEY=
NEXT_PUBLIC_FIREBASE_AUTH_DOMAIN=
NEXT_PUBLIC_FIREBASE_PROJECT_ID=
NEXT_PUBLIC_FIREBASE_STORAGE_BUCKET=
NEXT_PUBLIC_FIREBASE_MESSAGING_SENDER_ID=
NEXT_PUBLIC_FIREBASE_APP_ID=

Web Push certificates public key:

NEXT_PUBLIC_VAPID_KEY=

firebase service account private key

# firebase admin
FIREBASE_PRIVATE_KEY=

[MarconLP]

variables also have to be changed in https://github.com/MarconLP/loggl/blob/e64d5770496d73de5ced557933aa0f6bb0678ca4/apps/nextjs/public/firebase-messaging-sw.js, while this issue is not fixed: #7