Split utils tutorials into Body class and unit conversions

[emolter]

Describe the problem
The usage, and even perhaps existence, of the Body class is hidden from new users because it sits underneath a "utils tutorial" dropdown menu. It isn't clear what the utils tutorial will contain.

Propose a solution
Split the Body class tutorial and unit conversions tutorials into their own notebooks. Add more examples of how to use the Body class, and how it can work together with navigation and other modules.

Readthedocs page URL
https://pylanetary.readthedocs.io/en/latest/utils-tutorial.html