Document how to disable Google Chat/Meet in Mail

Why is this change being made?

My Google Mail greeted me with a message that Chat is now enabled, and the left mailbox folder view lost 50% of the space for Chat. Since I already have Google Meet disabled for the same reason, this MR is to document how to disable both integrations in Google Mail. You can still use them in separate windows where they do not clutter the mail overview.

Before

After

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