Docs add: filter users by assigned seat

What does this MR do and why?

Updates subscription add-on docs to explain how to view who is assigned a Duo seat by filtering users by assigned seat in the UI.

• Video showing how this works: &14683 (comment 2152650061)
• Relevant epic: [Duo Provisioning] Iteration 2: Add ability to ... (&14683)

This is new functionality released 2024-10-11 for GitLab.com and expected to make it into the 17.5 release for SM, but it may miss and move to 17.6 (at which point we will need to update the version requirement in this MR).

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