Clarify PolyScope X support

[urfeex]

Throughout the docs we have marked PolyScope X support preliminary, e.g. here or here.

From my understanding, the ExternalControl URCapX is now in a state where we could remove the beta statements, would you agree @urrsk @urmahp?

[urrsk]

I will say we need to have the External control for Polyscope X to support multiple external control nodes in one program before we can remove the beta flag

[urmahp]

I wouldn't consider the URCap as under development any longer, so I would either delete the last comment or just write, that you would need a different URCap for PolyScopeX.

Do we consider to make any breaking changes to the URCap? Because if we are not I would consider removing the beta flag and then just list the features that isn't supported yet.