Add doc page about joint kinematics

[christophfroehlich]

I summarized some results of discussions about mimic joints and how I understand that. Note, this is different than implemented currently with https://github.com/ros-controls/ros2_control/blob/16fbde3d7ee9e6d7ce19c789887eb4558118f59b/hardware_interface/src/mock_components/generic_system.cpp#L568-L576 This came from the time where we defined the mimic joints in the ros2_control tag. IMHO, it does not make sense now using the URDF-specification, where the behavior of the mimic joint changes by switching the command interface from position to velocity, because we use the offset and calculate velocity = multiplier * other_joint_velocity + offset

Also related to https://github.com/ros-controls/gazebo_ros2_control/pull/297 and https://github.com/ros-controls/gz_ros2_control/pull/276

I also included transmission interfaces, to give an overview what they are and what they aren't (they were not mentioned once in the docs).

[codecov[bot]]

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- Coverage   87.70%   62.67%   -25.04%     
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- Hits         7632     7089      -543     
+ Misses        790      715       -75     
- Partials      280     3507     +3227     

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