Add Marantz Infrared integration documentation

[balloob]

Proposed change

Adds documentation for the new marantz_infrared core integration. The integration controls a Marantz amplifier through an existing infrared transmitter entity (e.g. an ESPHome IR blaster).

The page documents:

• Setup via the config flow (model selector + transmitter entity).
• The media player entity (turn on/off, volume up/down, mute, source select with CD, Coax, Network, Optical, Phono, Recorder, Tuner).
• The button entities for amplifier-only commands (Speaker A/B, Source Direct, Loudness).
• Known limitations (assumed state, single power toggle, step-based volume, optical-input toggle behaviour).

Note

Initial support is intentionally limited to the Marantz PM6006. The page invites the community to file issues with captured IR signals to help expand support to other Marantz amplifier models.

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 Marantz IR Remote integration (PM6006, media player only) core#169626
• Link to parent pull request in the Brands repository: Add marantz_infrared symlink to existing marantz brand brands#10237
• 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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