Add user count info to internal support page

Why is this change being made?

This section is being added due to the large influx of questions that Support receives from sales regarding how a customer can obtain their total user count.

This MR also adds a small note that chatops oncall commands can be run in a DM with GitLab Chatops.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what