Improve docs for authentication via LDAP

Following on from work in RHODS-1757:

The current "Adding users for OpenShift Data Science" assembly does not provide sufficient information for easily configuring LDAP as the identity provider.

We already link to LDAP configuration and LDAP syncing to ensure customers have the information they need to sync an LDAP group to OpenShift.

• https://docs.openshift.com/container-platform/4.8/authentication/identity_providers/configuring-ldap-identity-provider.html
• https://docs.openshift.com/container-platform/4.8/authentication/ldap-syncing.html

We need to create an assembly for people who want to configure LDAP auth that includes all of the required steps, including changing rhods-groups-config to use the RHODS user group that is synced from LDAP. This currently (1.1 GA) requires cluster-admin permissions in OpenShift, but in future (???) will only require dedicated-admin permissions.