Document supervisor's new /auth/list endpoint

[mdegat01]

Proposed change

Document new endpoint created for home-assistant/supervisor#4912

Type of change

• Document existing features within Home Assistant
• Document new or changing features which there is an existing pull request elsewhere
• Spelling or grammatical corrections, or rewording for improved clarity
• Changes to the backend of this documentation
• Removed stale or deprecated documentation

Additional information

• This PR fixes or closes issue: fixes #
• Link to relevant existing code or pull request: Allow listing of HA users via admin CLI supervisor#4912

[mdegat01]

I didn't document the response model intentionally though I can if we want. My thinking was this:

1. The only intended use case of this API is user trying to recover their credentials on the host shell. They don't really need to know the response model to understand what they're seeing, it'll be clear
2. Supervisor doesn't dictate the response model. We're just calling Home Assistant's WS API for listing users and returning what we get. So it could change without Supervisor's knowledge and that's ok (see point 1)

[mdegat01]

After further review, changed my mind on the above. We wanted to filter out the system generated users and info to streamline it a bit. Since supervisor is now massaging the data model, documenting it here makes sense.