bug: no documentation for VirtualWorkspaces yet

[xrstf]

Describe the bug

We do not yet provide any user-facing documentation for the newly merged VirtualWorkspace support in the operator. Instead we merged a "mostly working with some caveat" state and left the rest for later (clbuttic).

Steps To Reproduce

1. Write documentation on how to use the new VirtualWorkspace objects.

Ensure to note

• that CacheServers are basically mandatory (in some weird scenarios it might work without a cacheserver, but in general, to prevent hours of fighting bugs, we should strongly urge users to deploy a CacheServer).
• support for 3rd party VWs is still limited (you can change the image, but your image then also has to take and deal with all of the CLI flags provided by the operator). We want to improve this and are very much open for feedback from folks running their own VWs.
• VWs can be mixed (some shards can have built-in VWs, some shards have external VWs).
• VWs have to be managed manually because we wanted to keep it as basic as possible and maybe build higher-level abstractions (VWSets?) later.

Expected Behaviour

N/A

Additional Context

No response