Feature: Add OnlyOffice as a subchart

[jessebot]

Description of the change

Let's add support for OnlyOffice as a subchart. Here's their helm chart.

• Split from #214

Benefits

Allow users to have a document server deployed in this stack.

Possible drawbacks

• Another thing to support.
• Unknown: Not sure how much we can facilitate actually configuring the connection between Nextcloud and OnlyOffice 🤷

Happy to discuss anything further!

Tasks

These are the tasks that would need to be done for this issue to be considered complete:

• [ ] add tests in ci and/or helm unit-tests
• [ ] add OnlyOffice as a configurable sub-chart via parameters in values.yaml
• [ ] install the OnlyOffice connector app

We can add more tasks if needed.