Add sensor platform to fressnapf_tracker

[eifinger]

Proposed change

Document battery sensors

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: Add sensor platform to fressnapf_tracker core#157658
• 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 adds documentation for the battery sensor platform to the Fressnapf Tracker integration, expanding the documented functionality beyond the existing device tracker.

Key changes:

• Added sensor to the list of supported platforms in the YAML front matter
• Documented the Battery sensor entity that shows the current battery level of the tracker

[c0ffeeca7]

Thank you, @eifinger 👍