docs: Update iZone integration documentation for multi-controller config flow support

[Swamp-Ig]

Proposed change

This updates the iZone integration documentation to match the new config flow behavior for multiple discovered controllers.

• Clarifies that config flow now discovers multiple controllers and lets you choose which one to set up
• Clarifies that additional controllers can be added by running setup again
• Keeps YAML exclude documented as a legacy/manual configuration path - note that I'm going to deprecate this code path as it's no longer really needed, and the implementation is flakey and not really consistent with best practice.

Type of change

• Spelling, grammar or other readability improvements (current branch).
• Adjusted missing or incorrect information in the current documentation (current branch).
• Added documentation for a new integration I'm adding to Home Assistant (next branch).
  • I've opened up a PR to add logos and icons in Brands repository.
• Added documentation for a new feature I'm adding to Home Assistant (next branch).
• Removed stale or deprecated documentation.

Additional information

• Link to parent pull request in the codebase: Refactor iZone config flow to on-demand discovery and handling multiple devices core#169251
• Link to parent pull request in the Brands repository:
• This PR fixes or closes issue: fixes #

Checklist

• This PR uses the correct branch, based on one of the following:
  • The next branch if it documents a new or changed feature in Home Assistant
  • The current branch if it corrects current documentation

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[Copilot]

Pull request overview

Updates the iZone integration documentation to reflect config flow behavior when multiple controllers are present, and clarifies how to use YAML to exclude controllers when needed.

Changes:

• Document multi-controller discovery and how to add additional controllers via the config flow.
• Reframe YAML setup instructions as an exclusion-focused option and update the example comment.
• Clarify that the exclude option applies only to YAML configuration.