Add information about state_class to template sensor

[AllardKatan]

Proposed change

Type of change

• [ ] Spelling, grammar or other readability improvements (current branch).
• [x ] 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

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

Summary by CodeRabbit

• Documentation
  • Updated the description of the state_class property in sensor configuration to clarify its role in long-term statistics and impact on history visualization.