Reorganize the list of cache files used in CI

[seisman]

Many remote files are cached as GitHub Actions assets to be reused in CI workflows. These files are listed in the pygmt/helpers/caching.py file. See below:

pygmt/pygmt/helpers/caching.py

Lines 13 to 20 in f0d4d21

	datasets = [
	# Earth relief grids
	"@earth_gebco_01d_g",
	"@earth_gebcosi_01d_g",
	"@earth_gebcosi_15m_p",
	"@earth_relief_01d_p",
	"@earth_relief_01d_g",
	"@earth_relief_30m_p",

Currently, the list is a little messy. Some potential improvements:

1. Put some comments (e.g., "Names like @N35E135.earth_relief_03s_g.nc are for internal use only.") at the start of the list.
2. Remove some comments like "# Specific grid for 15s test"
3. Update the order of some entries. First categorize files into groups and sort alphabetically in each group.

Helps are welcome.

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