Update Facebook integration documentation

[thorsteneckel]

Hi @KoJie - could you please update the Zammad Admin documentation on how to integrate Facebook as a channel? We received several questions (https://github.com/zammad/zammad/issues/417 / https://community.zammad.org/t/facebook-app-fehlermeldung/155 ) and should update our documentation accordingly. You can find the new screenshots attached. Feel free to ask any questions.

Facebook Zammad.zip

### Tasks
- [x] Setup of new Zammad system
- [x] Check Meta documentation
- [x] Add a new app at Meta's developer page
- [x] Restructure documentation section / add limitations & prerequisites
- [x] Setup facebook channel in Zammad
- [x] Add screenshots

[MrGeneration]

I am currently moving / organizing a new order within system admin and admin documentation, thus the borders get clearer.

"Parent"-Issue to this is: https://github.com/zammad/zammad-documentation/issues/43

[BenTen]

Is there some plan to update the facebook doc with 2021 version? I try to set some config to add facebook chanel on my project, but seems very obscure how the new system works :o

[MrGeneration]

No ETA sorry. I've got too much laying on my desk and as facebook keeps changing their shit constantly it's honestly a big downer for me too.

So right now you're alone on that path I'm afraid - sorry.