Revise GHA automation logic and documentation to handle changes to the dtcenter.org website configuration

[JohnHalleyGotway]

Describe the Task

On Feb 12, 2026, the configuration of the dtcenter.org website was modified so that https://dtcenter.ucar.edu/dfiles/ is no longer navigable, following security best practices. This change impacts the METplus automation logic, User Documentation, Contributors Documentation, and/or tutorial instructions. This issue is to address and resolve each of the impacts listed below:

• The Obtain Sample Input Data section of the METplus User's Guide contains a broken link to sample input data.
• The GHA script to search for tgz files that start with "sample_data"
  in https://dtcenter.ucar.edu/dfiles/code/METplus/test_data/METplus/develop/ no longer works.
• The GHA script to find new input data for MET does not work: https://dtcenter.ucar.edu/dfiles/code/METplus/test_data/MET/develop/
• Grep through the User's Guides and Contributor's Guides and .github directories for all METplus components, including METplus-Training looking for "dfiles" links to directories rather than specific files. Those would likely all now be broken.
• [ ]

Time Estimate

Unknown

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Define Related Issue(s)

Consider the impact to the other METplus components.

• METplus, MET, METdataio, METviewer, METexpress, METcalcpy, METplotpy

Task Checklist

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• Complete the issue definition above, including the Time Estimate and Funding Source.
• Fork this repository or create a branch of develop.
  Branch name: feature_<Issue Number>_<Description>
• Complete the development and test your changes.
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• Add any new Python packages to the METplus Components Python Requirements table.
• For any new datasets, an entry to the METplus Verification Datasets Guide.
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  Select: METplus-Wrappers-X.Y.Z Development project for development toward the next official release
• Iterate until the reviewer(s) accept and merge your changes.
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• Close this issue.