DOC: Document how to use CRDS to grab latest data files for HST

[pllim]

It seems that CRDS now supports "synphot" files in its system to the extend of HST throughput files.

Relevant links:

• https://hst-crds.stsci.edu/static/users_guide/command_line_tools.html?highlight=synphot
• https://hst-crds.stsci.edu/ (expand the "synphot" drop-down menu)

However, a user-friendly documentation on how to grab only those files (instead of a textbook-like user manual) and how to then tell stsynphot to use them is missing. I am thinking of something like a quick-start guide. And we can possibly document it here (along with the setup doc) or a new section in CRDS manual.

@rizeladiaz or @jaytmiller , do you know someone who can help me write such a thing? Thanks!

EDIT: Depends on spacetelescope/crds#731

[tddesjardins]

Some information is contained within the crds code help. The get_synphot routine can be used to download the synphot throughput files (not calspec) directly from the CRDS server. The crds package comes with a default astroconda installation. Type crds get_synphot --help for more information.