Add documentation for contacts organization chart

[kesselb]

• How to define the manager for a user
• How to set job title, role, etc
• The organization chart only works with accounts in the system addressbook

[kesselb]

cc @bkrith

[Brijeshthummar02]

@kesselb show me the path where should i add it.

[kesselb]

I think the documentation for contacts is the right place.

Here's the online version: https://docs.nextcloud.com/server/latest/admin_manual/groupware/contacts.html

Path in the repo: https://github.com/nextcloud/documentation/blob/master/admin_manual/groupware/contacts.rst