Fix issue #24286: document admin-settings.json sign-in enforcement (#24444)

Documents that the presence of admin-settings.json file enforces sign-in
on Docker Desktop.

Changes:
- Added important note in Prerequisites section explaining that the
file's presence enforces sign-in (behavior since Docker Desktop 4.29)
- Clarifies this is intended for business features that require
authentication

This addresses user confusion about why Docker Desktop requires sign-in
after adding the admin-settings.json file.

Fixes #24286

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Co-authored-by: David Karlsson <dvdksn@users.noreply.github.com>

Author: dvdksn