Add sensor documentation for NuHeat integration

[eric]

Proposed change

The Nuheat API has the number of minutes the floor was on, which can be used to calculate an estimated energy usage. If the settings have been set on the Nuheat server, it will use that to calculate the energy usage, but if not, they can be provided in Home Assistant settings.

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 minutes and energy sensors to Nuheat core#160751
• 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 new sensor functionality in the NuHeat integration. The changes document two new sensors (heating time and energy consumption) and explain how to configure the integration to calculate energy usage from floor specifications.

Changes:

• Added sensor platform support to the integration metadata
• Documented two new sensors: heating time and energy consumption
• Added configuration instructions for calculating energy from floor specifications

[Copilot]

The sentence 'lets you control' should flow better. Consider rephrasing to: 'The NuHeat {% term integration %} allows you to control your connected...' for a more natural, conversational tone that aligns with the friendly writing style.

[Copilot]

The formula should explain what the division operations represent for clarity. Consider rephrasing to: 'When floor area is configured, energy is calculated as: floor_area × watt_density × heating_minutes / 60 (minutes to hours) / 1000 (watts to kilowatts)' or adding a brief explanation after the formula.

Suggested change

	When floor area is configured, energy is calculated as: `floor_area × watt_density × heating_minutes / 60 / 1000`.
	When floor area is configured, energy is calculated as: `energy_kwh = floor_area × watt_density × heating_minutes / 60 / 1000`, where `/ 60` converts minutes to hours, and `/ 1000` converts watts to kilowatts.