Include registry examples in pysat tutorial

[aburrell]

Is your feature request related to a problem? Please describe.
I frequently forget how to register instruments in pysat, since I don't do it often. The first place I look is in the pysat docs tutorial (basics and params), but nothing there is helpful. Instead, I always end up going to the index, searching for register, and ending up in the API section.

Describe the solution you'd like
An example of how to register instruments in the tutorial. Ideally in either basics or params, since I usually look there.

Describe alternatives you've considered
Adding a link to the API in the tutorial, since the API example is fine for me. But this might not be clear for other users.

Additional context
Registration is an important step and one that isn't performed frequently so needs clear and easy to find documentation.

[rstoneback]

A link to the registry and an example was added to tutorial_basics.