Composition and Bridge Updates

[Amronos]

🦟 Bug fix

Summary

Updated link to ROS documentation for composition in ros2_overview.md.
Updated ros2_integration.md to add info about the new create_own_container argument.
Updated ros2_launch_gazebo.md to add documentation for gazebosim/ros_gz#620 and to add documentation for launching gazebo with ros_gz_bridge.
Changed the launch file used in ros2_spawn_model.md to gz_spawn_model.py instead of ros_gz_spawn_model.py. Added info about spawning a model alongside launching a bridge.

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)
• 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.

[caguero]

These are great updates!