Create tutorial explaining how visualize models spawned with ROS on GzWeb

[osrf-migration]

Original report (archived issue) by Louise Poubel (Bitbucket: chapulina, GitHub: chapulina).

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This is a common pain point for users, see these issues for example:

• https://bitbucket.org/osrf/gzweb/issues/86/not-able-to-visualise-ur10-in-gzweb
• https://bitbucket.org/osrf/gzweb/issues/78/spawning-the-pr2-robot-vs-inserting-the