Add Fuelprices.dk documentation

[MTrab]

Proposed change

This PR adds documentation for the new dk_fuelprices integration (Fuelprices.dk).

The page includes:

• Integration overview and API/source description
• Prerequisites and account/API key steps
• Standard config flow include
• Standard removal section
• Link to currently supported companies on Fuelprices.dk
• Metadata aligned with the integration (ha_domain, iot class, platforms, quality scale, codeowner, and integration type)

This documentation matches the implementation added in core:
home-assistant/core#163932

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 Fuelprices.dk core#163932
• Link to parent pull request in the Brands repository: Move dk_fuelprices to core integrations brands#9826
• 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.

[c0ffeeca7]

Thank you, @MTrab 👍