Update Document

Add an example for class Color as one goal mentioned in #300

Summary

Add an example for class Color and change the existing documents.

Test it

cd example
mkdir build
cmake ../
make
./color_example

Others

I found something weird about the ignition::math::Color::HSV() and ignition::math::Color::SetFromHSV() when writing the example. When using the return value of the HSV() as the input for SetFromHSV(), the value of the instance is inconsistent. But I am not sure if it is a bug or I'm just using it the wrong way so I am not sure if making it a separate issue is a good idea.

Like this (put it in /example)

#include <iostream>
#include <ignition/math/Color.hh>

int main() 
{
  ignition::math::Color a = ignition::math::Color(0.5, 0.6, 0.7);
  std::cout << "The RGBA value of a: " << a << std::endl;
  std::cout << "The HSV value of a: " << a.HSV() << std::endl;
  
  // use its own HSV value to set HSV value, should be exactly the same.
  a.ignition::math::Color::SetFromHSV(a.HSV()[0], a.HSV()[1], a.HSV()[2]);
  std::cout << "The RGBA value of a: " << a << std::endl;
  std::cout << "The HSV value of a: " << a.HSV() << std::endl;

  // one more time
  a.ignition::math::Color::SetFromHSV(a.HSV()[0], a.HSV()[1], a.HSV()[2]);
  std::cout << "The RGBA value of a: " << a << std::endl;
  std::cout << "The HSV value of a: " << a.HSV() << std::endl;
}

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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