Add platform waterheater

[liudger]

Proposed change

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 waterheater platform bsblan core#129053
• 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.

Summary by CodeRabbit

• New Features
  • Enhanced BSB-Lan integration documentation to include "Water heater" as a new category.
  • Expanded platform support with the addition of "water_heater" for improved device compatibility.
• Documentation
  • Streamlined documentation by removing duplicate lines related to config flow.