Expand LDAP howto docs

What does this merge request do and why?

• The first time that I set up LDAP I was confused about how to set up sync
• Also john is a member of group2, not mary: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/0b568478ad57a020a3618ed3c5aadfbaf5d0a7df/gitlab-openldap/frontend.example.com.ldif#L109
• Also updates docker command for SAML to include --platform arg because without that I get the error: "WARNING: The requested image's platform (linux/amd64) does not match the detected host platform (linux/arm64/v8) and no specific platform was requested".

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.