Pick and place tutorial was failing to compile its scripts because
the package manifest was pointing to an older version of
ROS-TCP-Connector.  Updated the manifest and also pulled latest changes
for ros_tcp_endpoint.

Tested by running DemoScene

* Contributing content

* PR feedback

* added example of implementing a UnityService

* adding tutorial for unity service

* added a couple comments to the example code

* typos and code standard fixes

* consistency: changing the service name for position_service to pos_srv

* updating the tutorial to match the corresponding script

* add new sections for the released new physics features

Co-authored-by: Amanda <31416491+at669@users.noreply.github.com>