<workspaceid> placeholder for workspace namespace/project

[sparkoo]

### Is your enhancement related to a problem? Please describe.
namespace/project can have placeholders now (#13488). Empty value would mean that workspace.id will be used as a namespace/project. It would now make sense to have <workspace.id> placeholder.

### Describe the solution you'd like
<workspace.id> placeholder for namespace/project.
https://github.com/eclipse/che/blob/master/assembly/assembly-wsmaster-war/src/main/webapp/WEB-INF/classes/che/che.properties#L375
https://github.com/eclipse/che/blob/master/assembly/assembly-wsmaster-war/src/main/webapp/WEB-INF/classes/che/che.properties#L581

### Additional context
Namespace placeholders was introduced as a fix for this #13488. Work on this after PR is merged #14524

As a consequence, PR #14828 is adding support for <workspaceid> in the namespace configuration. This issue is for updating the docs with the new information.

[l0rd]

Setting P1 to this issue because it looks necessary if we want to continue supporting one namespace per workspace strategy (even if the default is one namespace per user).

[sparkoo]

I think it is already part of PR #14828 -> https://github.com/eclipse/che/pull/14828/files#diff-4eed0e9903e80a21758bbe991eee8c37R66
@metlos ?

[metlos]

I think it is already part of PR #14828 -> https://github.com/eclipse/che/pull/14828/files#diff-4eed0e9903e80a21758bbe991eee8c37R66
@metlos ?

Yes, #14828 is adding this functionality. I've updated the description of this issue to be basically docs-only.