TrackedVehicle: Added docs for body_link option

[peci1]

🦟 Bug fix

Fixes #2650

Summary

This PR adds forgotten documentation for the <body_link> tag in TrackedVehicle system.

The docs refer to source code at https://github.com/gazebosim/gz-sim/blob/gz-sim9/src/systems/tracked_vehicle/TrackedVehicle.cc#L238 and https://github.com/gazebosim/gz-sim/blob/gz-sim9/src/systems/tracked_vehicle/TrackedVehicle.cc#L475 .

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[ahcorde]

@osrf-jenkins run tests please!

[iche033]

the Examples test failure should be fixed once #2649 is merged forward to main. The homebrew test failures are due to osrf/homebrew-simulation#2834. This PR should be safe to merge since it just adds a comment.