Documentation: Extend/improve documentation for graphical user management.

Summary

https://docs.indiscale.com/caosdb-deploy/configuration.html#graphical-access-control-management only mentions that envoy is required but is lacking the information on port and URI of the ACM. Also, it should explain BUILD_MODULE_EXT_ENTITY_ACL_USER_MANAGEMENT_BUTTON and its difference to BUILD_MODULE_EXT_ENTITY_ACL.

Explain missing parts and add screenshots as explanations for the two build variables.