Extended documentation of ghost

[tim-griesbach]

Extended documentation of ghost

This PR extends and adds documentation on the ghost layer in p4est. The concrete changes are

• a new Doxygen page on the ghost layer that gives a general introduction (in parts based on https://p4est.org/tutorial-ghost.html),
• add and reference existing examples for ghost,
• fix all Doxygen warnings in p{4,8}est_ghost.h and
• extend the documentation on piggies in the p{4,8}est_quadrant_data and clarify the role of the piggies.

[cburstedde]

Thanks, looks great. I like the extra doc page a lot, and thanks for going after the warnings.