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

• [x] Signed all commits for DCO
• [ ] Added tests
• [x] 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)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[ahcorde]

@osrf-jenkins run tests please!

[iche033]

the Examples test failure should be fixed once https://github.com/gazebosim/gz-sim/pull/2649 is merged forward to main. The homebrew test failures are due to https://github.com/osrf/homebrew-simulation/issues/2834. This PR should be safe to merge since it just adds a comment.