Add a tutorial about coordinate system conventions

[leouieda]

We try to use a consistent convention for coordinate systems across our projects. It would be great to have a central tutorial here that can be referenced in the other documentation sites.

The tutorial could be a mix of the one @santisoler wrote for Harmonica (https://www.fatiando.org/harmonica/dev/user_guide/coordinate_systems.html) and the one @LL-Geo wrote for Boule (https://www.fatiando.org/boule/v0.3.1/tutorials/geodetic_to_geocentric.html; but using pymap3d instead of Boule for the operations).

"Some more content that can be copied: https://tesseroids.readthedocs.io/en/stable/theory.html"

See discussion in fatiando/boule#31

Should be done after #6 is finished.