Add timestamp to TwistStamped msg (backport #828)

[mergify]

Main changes
When following the steps for example_11, I observed lot of warnings logged to the console due to missing timestamps in the header of the TwistStamped message.

This PR updates the instructions and modifies the relevant examples to include timestamps in the TwistStamped message and eliminates the warnings.

[ros2_control_node-1] [WARN] [1749236154.127088701] [bicycle_steering_controller]: Timestamp in header is missing, using current time as command timestamp.
[ros2_control_node-1] [WARN] [1749236154.160455387] [bicycle_steering_controller]: Timestamp in header is missing, using current time as command timestamp.
[ros2_control_node-1] [WARN] [1749236154.193923786] [bicycle_steering_controller]: Timestamp in header is missing, using current time as command timestamp.
[ros2_control_node-1] [WARN] [1749236154.227070187] [bicycle_steering_controller]: Timestamp in header is missing, using current time as command timestamp.
...
[ros2_control_node-1] [WARN] [1749236154.594182149] [bicycle_steering_controller]: Timestamp in header is missing, using current time as command timestamp.

---

This is an automatic backport of pull request #828 done by [Mergify](https://mergify.com).

[mergify]

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Changes to be committed:
	modified:   example_11/doc/userdoc.rst
	modified:   example_2/doc/userdoc.rst

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   example_16/bringup/launch/demo_test_helper.py
	deleted by us:   example_16/doc/userdoc.rst

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