Hi! I have created a "decision tree" but this is also my first issue so I am not sure how/where should I add it. Can I just add it to numpy/doc/source/reference/figures/ ? Thank you for your answer.

You could put it next to the howto itself in doc/source/user and reference it with .. image:: filename

It would be great if you could add a complete guide how you generated it in case someone wants to update it, extra points for using a scripting solution to make it reproducable. Also see the howto document document

I'd caution marking this "good first issue", else we might end up with a thread similar to sympy/sympy#15331.