Docs for new Ruuvi Gateway integration

[akx]

Proposed change

Docs for home-assistant/core#84853.

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:
    Convert ruuvitag_ble to a brand used for both ruuvi_gateway and ruuvitag_ble brands#3984
• 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: Ruuvi Gateway integration core#84853
• Link to parent pull request in the Brands repository: Convert ruuvitag_ble to a brand used for both ruuvi_gateway and ruuvitag_ble brands#3984
• This PR fixes or closes issue: fixes #

Checklist

• This PR uses the correct branch, based on one of the following:
  • 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.

[frenck]

✅ Approved. Can be merged as soon as the parent PR gets merged.
⚠️ The ha_release might need an update before merging this one in.

[frenck]

Thanks, @akx 👍