Added example world for `DopplerVelocityLogSystem`

[sauk2]

🎉 New feature

Closes #2344

Summary

The examples/worlds directory does not contain any example world for DopplerVelocityLogSystem plugin. This PR adds the following:

• Example world based on test/worlds/flat_seabed.sdf but with a SceneBroadcaster.
• README.md in test/worlds directory to inform users not to use those world files.

More information can be found in Issue #2344 and PR #2346.

Test it

This world can be tested (after building and sourcing the workspace) using the following command

  gz sim flat_seabed.sdf

Checklist

• [x] Signed all commits for DCO
• [x] Added example and/or tutorial
• [x] Updated documentation
• [ ] codecheck passed (See contributing)
• [ ] All tests passed (See test coverage)

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.