Add User's Guide documentation on how to use the plotting scripts in the workflow

[gspetro-NOAA]

Description

Currently, the ReadTheDocs (RTD) documentation does not reflect the addition of the plot_allvars task to the workflow in PR #482.

Solution

Update the following files to reflect the changes made in PR #482 :

• RunSRW.rst --> Update the plotting section to reflect that plotting scripts are now part of the workflow. Migrate relevant information from the Graphics chapter to the RunSRW chapter.
• Delete the Graphics.rst file once relevant info is incorporated into RunSRW.rst.
• ConfigWorkflow.rst --> Add variables and descriptions from the config_defaults.yaml documentation to this file.