Document enabling Group SAML on an instance, including why not to

What

Documents how Group SAML could be enabled on a GitLab instance, and describes why we don't yet recommend this in most cases.

Overlaps with https://gitlab.com/gitlab-org/gitlab-development-kit/blob/master/doc/howto/saml.md, where we have developer documentation.

Why

We have some customers asking how to enable this since it is listed as a premium feature. We'll likely unlist it as a premium feature because it is primarily aimed at GitLab.com, but should document why the instance-wide integration should usually be preferred. Additionally we should document how to configure it for organizations that understand the limitations and wish to use it.