docs(joint_state_broadcaster): clarify /dynamic_joint_states contents

[rishitej04]

This PR updates the existing doc/userdoc.rst for the joint_state_broadcaster.

What’s included

• Added a new "Published topics" section:
  • /joint_states: publishes movement-related interfaces (position, velocity, effort).
  • /dynamic_joint_states: publishes all available state interfaces per joint,
    including custom ones (e.g., temperature, voltage, torque sensor readings).
• Added a YAML-style example payload for clarity.
• Added a note about the use_local_topics parameter.

Why

• The previous documentation only stated that both topics are published but did not
  explain their contents in detail.
• This clarification makes the documentation consistent with demos and API expectations
  and answers common questions from new users.

Fixes #1716

[christophfroehlich]

there is already a userdoc.rst file in the doc folder. please update it instead.

[rishitej04]

yep done have a look at it and let me know

[bmagyar]

thank you!