Add example and troubleshooting to Powerfox integration docs

[klaasnicolaas]

Proposed change

As requested by the new quality scale

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:
• 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:
  • I made a change to the existing documentation and used the current branch.
  • I made a change that is related to an upcoming version of Home Assistant and used the next branch.
• The documentation follows the Home Assistant documentation standards.

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[Copilot]

Pull request overview

This PR enhances the Powerfox integration documentation by adding an example automation and expanding the troubleshooting section, as requested by the quality scale requirements.

• Adds a practical automation example showing how to get alerts for high power usage
• Fixes a minor grammar issue in the introduction (changing "fetch" to "fetching")
• Corrects a typo in the water sensor section ("eneregy" → "energy")
• Replaces the generic troubleshooting statement with specific guidance about cost sensors