website/docs: document the Password Uniqueness Policy

[verkaufer]

Relates to https://github.com/goauthentik/authentik/pull/10631

Details

Introduces public documentation for the Unique Password Policy feature added by https://github.com/goauthentik/authentik/pull/10631

Documentation changes were move into a separate PR to keep feedback conversations focused on the specific changes.

👉 This PR should not be merged before https://github.com/goauthentik/authentik/pull/10631.

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Checklist

• [x] Local tests pass (ak test authentik/)
• [x] The code has been formatted (make lint-fix)

If an API change has been made

• [ ] The API schema has been updated (make gen-build)

If changes to the frontend have been made

• [ ] The code has been formatted (make web)

If applicable

• [x] The documentation has been updated
• [x] The documentation has been formatted (make website)