Add HmIP-WGC to homematicip_cloud integration

[hahn-th]

Proposed change

Add documentation for button HmIP-WGC to homematicip_cloud integration.

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 logo's and icons in Brands repository.
• [X] 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: #75733
• Link to parent pull request in the Brands repository:
• This PR fixes or closes issue: fixes #

Checklist

• [X] 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.
• [X] The documentation follows the Home Assistant documentation standards.