Document known issues with the Unraid platform

## Description

Multiple users report issues running Graylog on the [Unraid](https://unraid.net/) platform. There is a filesystem-related issue with Graylog's message journal and the Unraid system. We tried to reproduce the problem by setting up test environments with the Unraid platform without success. See issue https://github.com/Graylog2/graylog2-server/issues/12949 for more details.

Since we cannot reproduce the issue and the Unraid system is mainly designed to run on home media setups, we should add a warning to our documentation that we don't recommend and support running Graylog on Unraid systems for now.

The mentioned GitHub issue includes a workaround, but we don't recommend it because it increases the chance of losing messages.