Feature #2207 Documentation - Updates to Contributor's Guide

[georgemccabe]

Pull Request Testing

• Describe testing already performed for these changes:
  

N/A

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

Review changes to docs:

• https://metplus.readthedocs.io/en/feature_2207_doc_contrib_updates/Contributors_Guide/add_use_case.html#add-volume-mount-directories-file
• https://metplus.readthedocs.io/en/feature_2207_doc_contrib_updates/Contributors_Guide/add_use_case.html#copy-data-from-the-feature-directory-into-the-next-version-directory
• https://metplus.readthedocs.io/en/feature_2207_doc_contrib_updates/Contributors_Guide/github_workflow.html#open-a-pull-request

• 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]

N/A

• Will this PR result in changes to the test suite? [No]
  

• Please complete this pull request review by 7/28/2023.
  

Pull Request Checklist

See the METplus Workflow for details.

• Add any new Python packages to the METplus Components Python Requirements table.
• 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)
  Select: Organization level software support Project or Repository level development cycle Project
  Select: Milestone as the version that will include these changes
• After submitting the PR, select Development issue with the original issue number.
• 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.

[j-opatz]

The changes made seem to clear up a few confusing commands from the original material, and correct at least 1 spelling mistake.

The PR is approved.

[DanielAdriaansen]

Just one small change to force the copy lines to occur in a code block otherwise looks great. Thanks in particular for adapting the github workflow section to better distinguish between branches and forks. It's much improved IMHO @georgemccabe!