Add GPIO tag description to docs

[christophfroehlich]

As requested with ros-controls/control.ros.org#57 I propose adding a new page to the docs describing joints, sensors, and gpios hardware_interface types as used in

ros2_control/hardware_interface/include/hardware_interface/hardware_info.hpp

Lines 118 to 132 in c122455

	/**
	* Map of joints provided by the hardware where the key is the joint name.
	* Required for Actuator and System Hardware.
	*/
	std::vector<ComponentInfo> joints;
	/**
	* Map of sensors provided by the hardware where the key is the joint or link name.
	* Required for Sensor and optional for System Hardware.
	*/
	std::vector<ComponentInfo> sensors;
	/**
	* Map of GPIO provided by the hardware where the key is a descriptive name of the GPIO.
	* Optional for any hardware components.
	*/
	std::vector<ComponentInfo> gpios;

I added a link to ros-controls/ros2_control_demos#256 assuming it to be merged before this one.

[codecov-commenter]

Codecov Report

Merging #1109 (1471772) into master (925f5f3) will decrease coverage by 2.79%.
Report is 513 commits behind head on master.
The diff coverage is 31.53%.

❗ Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the GitHub App Integration for your organization. Read more.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1109      +/-   ##
==========================================
- Coverage   34.61%   31.83%   -2.79%     
==========================================
  Files          52       94      +42     
  Lines        2981    10475    +7494     
  Branches     1855     7130    +5275     
==========================================
+ Hits         1032     3335    +2303     
- Misses        310      801     +491     
- Partials     1639     6339    +4700     

Flag	Coverage Δ
unittests	31.83% <31.53%> (-2.79%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
controller_manager/src/controller_manager.cpp	38.56% <ø> (-1.15%)	⬇️
controller_manager/src/ros2_control_node.cpp	0.00% <0.00%> (ø)
..._interface/include/hardware_interface/actuator.hpp	100.00% <ø> (ø)
...ce/include/hardware_interface/async_components.hpp	0.00% <0.00%> (ø)
...re_interface/include/hardware_interface/sensor.hpp	100.00% <ø> (ø)
...re_interface/include/hardware_interface/system.hpp	100.00% <ø> (ø)
...erface/src/mock_components/fake_generic_system.cpp	100.00% <ø> (ø)
...e_interface/src/mock_components/generic_system.cpp	52.29% <ø> (ø)
hardware_interface/src/resource_manager.cpp	46.76% <ø> (-6.87%)	⬇️
hardware_interface/src/sensor.cpp	50.52% <ø> (ø)
... and 68 more

... and 22 files with indirect coverage changes

[bmagyar]

Suggested change

	Parsing of ``<gpio>``-tag is similar to this of a ``<joint>``-tag having command and state interfaces.
	Parsing of ``<gpio>``-tag is similar to that of a ``<joint>``-tag having command and state interfaces.

[bmagyar]

Merged without this 🤦

[bmagyar]

Super nice, thank you!