rviz plugin tutorial

[LeweC]

It is currently very hard to find working examples or tutorials for custom rviz plugins.
As mentioned in #3242 and #3384

At some point I got lucky and found a very good repo for this, with many different kinds of rviz plugins. Also in many different ROS versions. But it was not easy to find.

I think it would make sense to include it here so that other people can find it more easily.

I would be happy to create a pull request if someone can give me some pointers where and how to include them here.

[clalancette]

I think it would be great to add in those examples somewhere. We just want to make sure that they are clearly marked as community-contributed examples, but otherwise please feel free to open a PR adding them. Thanks!

[DLu]

See also #3384