Add UnifiedPush settings in documentation

[mononym]

Hello.

I suggest to add an entry to the ntfy docs to explain how to make UnifiedPush work. Basically, one needs to apply this lines to the app/data/config/server.yml:

auth-access:
  - "everyone:up*:write-only"

c.f. https://docs.ntfy.sh/config/#example-unifiedpush

It could also be a setting applied by default.