Documentation revision: PAT notification emails

What does this MR do?

Enhances the documentation for the addition of instance-wide personal access token expiry in 12.6. !17344 (merged) added documentation about the admin setting, but it seems useful to document that this feature exists within the PAT documentation.

Adds documentation about the notification emails added by !19296 (merged).

And also mentions impersonation tokens.

Related issues

#3649 (closed)

Author's checklist (required)

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Review checklist

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