update doco for NRT global and nested

[DavidBurrows-NCO]

Description:

What: Add readTheDocs instructions on using the OMD global-EAGLE checkpoint file.
Why: needed for global-EAGLE configuration
Refs #123

Type of change:

• Bug fix
• Enhancement / new feature
• Refactor / cleanup
• Documentation
• CI/CD or tooling
• Subcomponent update
• Other:

Area(s) affected

• nested_eagle workflow
• Training
• Inference
• Verification / evaluation (via WXVX)
• Data prep / UFS2ARCO
• Config (YAML)
• Plotting / post-processing
• Infrastructure / Slurm scripts
• Other:

Commit Requirements:

• This PR addresses a relevant NOAA-EPIC/EAGLE issue (if not, create an issue); a person responsible for submitting the update has been assigned to the issue (link issue)
• Fill out all sections of this template.
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have made corresponding changes to the system documentation if necessary
• I checked whether this PR requires subcomponent PRs and completed the subcomponent checklist below

Testing / Verification:

• I ran and/or verified the changes (or provided a test plan)

Subcomponent Pull Requests:

• No subcomponent PRs are required for this change.

[github-actions]

Link to ReadTheDocs sample build for this PR can be found at:
https://epic-eagle--202.org.readthedocs.build/en/202

[github-actions]

Link to ReadTheDocs sample build for this PR can be found at:
https://epic-eagle--202.org.readthedocs.build/en/202

[github-actions]

Link to ReadTheDocs sample build for this PR can be found at:
https://epic-eagle--202.org.readthedocs.build/en/202

[github-actions]

Link to ReadTheDocs sample build for this PR can be found at:
https://epic-eagle--202.org.readthedocs.build/en/202