Document the distribution of GSHHS via auto-download on the server

Some time ago (2 years) we implemented a scheme wherein GMT would try to download the GSHG data set from the GMT server if the data were not installed on the user's system. The files are current (2.3.7) and stored in a version-less directory /export/gmtserver/gmt/data/geography/gshhg. The process works fine but there is on problem:

1. We have no reminders or guidelines to update these files.

The same setup applies to DCW and here we are behind on the version. We need to document this process here so that the next GSHHG is also placed on the server in the same directory.

Start by reviewing the existing GSHHG auto-download process described in this issue, including the GMT server directory /export/gmtserver/gmt/data/geography/gshhg and the parallel DCW setup. Document how updated GSHHG files should be placed there, how versions are tracked, and when maintainers should update them; done means the next update can follow the documented reminders and guidelines.

Written by the indexing model from the issue text.