Feature 2891 main v6.0 existing builds modulefiles

[jprestop]

Pull Request Testing

• Describe testing already performed for these changes:
  

Reviewed the formatting and the content of the Existing Builds section in RTD:
https://metplus.readthedocs.io/en/feature_2891_main_v6.0_existing_builds_modulefiles/Users_Guide/release-notes.html#existing-builds

Loaded the modulefiles on each machine to ensure they loaded properly.

• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
  

Quick review of Existing Builds page linked above to ensure there are no obvious errors.

• Do these changes include sufficient documentation updates, ensuring that no errors or warnings exist in the build of the documentation? [Yes]

• Do these changes include sufficient testing updates? [Yes]

• Will this PR result in changes to the test suite? [No]
  
  If yes, describe the new output and/or changes to the existing output:
  

• Do these changes introduce new SonarQube findings? [No]
  
  If yes, please describe:

• Please complete this pull request review by [ASAP].
  

Pull Request Checklist

See the METplus Workflow for details.

• Add any new Python packages to the METplus Components Python Requirements table.
• For any new datasets, an entry to the METplus Verification Datasets Guide.
• Review the source issue metadata (required labels, projects, and milestone).
• Complete the PR definition above.
• Ensure the PR title matches the feature or bugfix branch name.
• Define the PR metadata, as permissions allow.
  Select: Reviewer(s) and Development issue
  Select: Milestone as the version that will include these changes
  Select: Coordinated METplus-X.Y Support project for bugfix releases or METplus-Wrappers-X.Y.Z Development project for official releases
• After submitting the PR, select the ⚙️ icon in the Development section of the right hand sidebar. Search for the issue that this PR will close and select it, if it is not already selected.
• After the PR is approved, merge your changes. If permissions do not allow this, request that the reviewer do the merge.
• Close the linked issue and delete your feature or bugfix branch from GitHub.