Update della docs and minor edits to example

[laurejt]

Associated Issue(s): N/A

Changes in this PR

• Renamed della-instructions.md to della-guide.md
• Revised and expanded della-guide.md contents.
• Minor edits to slurm example translage-della.slurm:
  • Small fixes to the slurm GPU configuration to align with Della documentation
  • Updated args validation so the exact number of input parameters must be specified

Reviewer Checklist

• Check that della guide is correct and legible
• Check that all links in the guide work
• Confirm that the example script with the instructed updates runs for both CPU and GPU jobs.

[tanhaow]

Looks good 🚀

[tanhaow]

👍