Document steps to authorize "Account Level App"

[sadohert]

A customer was following the docs and ran into an issue with Zoom.

They are using the "Account Level App" integration (versus the "User Level").

We need to describe the authorization steps that a Zoom admin needs to follow to complete the integration. This issue comment explains the additional steps:

• On Mattermost, as a sysadmin, run /zoom connect
• Follow the link and follow the instructions

Not sure of the best place to organize this. Offhand I'd say one of these approaches:

1. An additional "Account Level App"-specific instruction under the "Mattermost Setup" page.
2. Under "Usage" section have a "Connect your account (User Level)", and separate "Connect your account (Account level)"