Clarify recommendation for external vs. internal usage of IDM service #1062

The DEV section of the IDM services currently says:

https://docs.opencloud.eu/docs/dev/server/services/idm/information

"It is mainly targeted at small OpenCloud installations. For larger setups it is recommended to replace IDM with a “real” LDAP server or to switch to an external identity management solution."

It might be useful to also have some recommendation within the ADMIN section which clarifies when it makes sense to strip the IDM service from the single binary integrated one and operate an external LDAP service.