Add more targets to doc/makefile help information

[maxrjones]

Description of the problem

The available targets in the docs Makefile are help, clean, html, api, linkcheck, doctest, and serve. The help target only lists information about the all, html, linkcheck, and doctest targets. One line each could be added to help to describe clean,api, and serve, for example @echo " clean clean up build and generated files".

[seisman]

We may want to remove the linkcheck target, because make linkcheck is so slow (> 5 minutes for me), and we will have a workflow for checking links (#634) (in process, but close to be ready).

[weiji14]

I never even realized we had a make linkcheck 😅 But yes, agree that this is a good first issue.

And just wanting to raise the issue of making things cross-platform (read: runs on Windows). Leo suggested using nox at #679 (comment) and fatiando/maintenance#8, but I'm wondering if our Makefile would just work on Windows if we install make from conda-forge?

[seisman]

@GenericMappingTools/python @GenericMappingTools/python-contributors @GenericMappingTools/python-maintainers

We are close to the v0.3.1 release, and I think we can wrap up this issue. Does anyone want to work on it?

[core-man]

@GenericMappingTools/python @GenericMappingTools/python-contributors @GenericMappingTools/python-maintainers

We are close to the v0.3.1 release, and I think we can wrap up this issue. Does anyone want to work on it?

I am working on it in #1014