Originally proposed in #2834:

Justification codes like BL and TR are commonly used in GMT and PyGMT. Currently, we have explanations like:

The justification is a two-character code that is a combination of a horizontal (L\ (eft), C\ (enter), or R\ (ight)) and a vertical (T\ (op), M\ (iddle), or B\ (ottom)) code.

Instead of repeatedly explaining justification codes in different plotting methods, I feel it makes more sense to have a separate page explaining justification codes then we can link to this page when necessary.