Document the scorecard <threshold_file> in the METviewer User's Guide

[JohnHalleyGotway]

Describe the Task

While working on scorecards for HAFS, @briannen needed to use the <threshold_file> scorecard option to modify colors, symbols, thresholds, and so on. While that XML tag is mentioned in the METviewer User's Guide, no description or example of its contents are provided.

This issue is to add a new section to the Scorecard chapter of the METviewer User's Guide about the <threshold_file> options. Please see examples from @CPKalb from 2020 (the last time this question came up) showing CAM scorecards in:
dakota:/d3/projects/USWRP_CAM/metv/thresh_test_sigdiff.xml

Recommend that @briannen do this work and request @bikegeek to review the updated content.

Time Estimate

Approx 4 hours

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Please coordinate with @KathrynNewman and @michelleharrold on funding through HAFS or other DTC keys.

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