Thread border router: add intro

[c0ffeeca7]

Proposed change

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:
• 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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Pull request overview

This PR adds an introductory sentence to the "About Thread border routers" section and refines existing content to improve clarity and readability for users learning about Thread border routers in Home Assistant.

• Adds a clear definition of Thread border router at the beginning of the section
• Improves clarity by specifying "IoT devices" and "radio chip" for better technical precision
• Adds an explicit statement about device control being handled by application layer protocols