Documentation and support scripts for supplements

**Description of the desired feature**

Looking for a solution to these two related documentation problems:

1. When supplemental program documentation need to include illustrations we currently end up placing new GMT scripts in the core doc/scripts. These are then run as part of the docs and tests and images are included in the eventual supplemental HTML file.
2. The GMT documentation that builds the supplemental modules depends on hardwired listings of supplemental modules (e.g., doc/rst/source/modules.rst). It would be better if that part was auto-generated from whatever supplements we have.

Thus, I would like to have more of a fire-wall between supplements and the core. The doc/scripts should really only be for GMT core documentation. For supplements, perhaps we could just place those scripts in the relevant supplement sub-directory, e.g, src/potential, and then Cmake needs to add those *.sh files to the list that needs to be processed. Currently we do not have much of this, but I started it with four scrips called GMT_seamount_*.sh that live in doc/scripts and are included in the supplements/potential/grdseamount.rst documentation, and I am about to add more for other tools. As for the documentation, these are currently separated from the supplemental src directories and instead are placed in doc/rst/source/supplements/subdir, i.e., inside the core doc tree. Perhaps a cleaner system would have those *.rst files with the rest of the supplement files in src/subdir and build the index from src/*/*.rst instead. That would make them truly _supplemental_.

Finally, while this approach would not change anything on the outside for our standard supplements, it would make it easier for us or other users to include _non-sanctioned_ GMT supplements (e.g., my **gsfml** or the **mbsystem** three modules) and have the documentation placed as well.

Wonder if @seisman and @joa-quim have suggestions for this.