Text processing docs for admins

[ChristophWurst]

Like https://github.com/nextcloud/documentation/pull/10714 but explain to admins what they need to do to get these shiny new features and integrations.

E.g. that they have to install the llm or integration_openai apps.

It will help admins install the new features they'll see in Hub 6 announcements.

Needs a backport to 27