Add more examples

[m-lundberg]

There is currently only one quite basic example to show how the PID controller should be used. It would be nice to have some more examples, perhaps also showing the more advanced/specialized features provided by the library (set_auto_mode(), error_map, the various possible values of output_limits etc). The examples should still be as short and to the point as possible while being complete and runnable.

[yashvarshney003]

@m-lundberg Can you elaborate. I want to do this.

[stonez56]

@m-lundberg Can you make an example of keeping an object is always centered in a video stream?