Update docs (backport #692)

[mergify]

IMHO, the section Default gz_ros2_control Behavior is completely wrong. Without a plugin tag, nothing will be started.

And I reformatted the URDF/SDF parts to use tabs.

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This is an automatic backport of pull request #692 done by Mergify.

[mergify]

Cherry-pick of 1f191aa has failed:

On branch mergify/bp/humble/pr-692
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	both modified:   doc/index.rst

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