Documentation of the xbudget dictionary structure and surrounding functions

[hdrake]

@jetesdal's main feedback (see NOAA-GFDL/xwmt#40 (review)) is that the xbudget dictionary data structure is a confusing black-box for new users. We need documentation to make it possible for new users to understand existing (preset) dictionaries (or .yaml convention files) or to make their own.

Thank you for @jetesdal for being an early tester of xbudget!

[hdrake]

xbudget now has some sparse documentation via https://xbudget.readthedocs.io/en/latest/, but it needs a tutorial (and maybe some helper functions) for new users to make their own budget .yaml files.