Improve "make clean"

[seisman]

Description of the problem

Running make clean can delete some temporary files and directories, but I usually have two problems with it:

1. make clean doesn't delete the files in doc/_build directory. IMHO, it should.
2. make clean deletes the .egg-info directory, so I have to reinstall pygmt in editable mode again (am I right here?)

Solution:

1. make clean should delete doc/_build directory but do NOT delete the .egg-info directory
2. Add a new target make dist-clean (or spotless or other better names) to delete all these files, including .egg-info directory.

Are you willing to help implement and maintain this feature?
Yes, but let's discuss it first.

[seisman]

https://www.gnu.org/software/make/manual/html_node/Standard-Targets.html

clean and distclean are the standard targets in Makefile. I think we can following the rules, and let make distclean delete the .egg-info diretcory.