Update UPC++ documentation

Dan Bonachearequested to merge
dobonachea/nersc.gitlab.io:2023-03-upcxx-update into main

Thank you for this contribution to docs.nersc.gov!

If non-trivial please provide a summary of the changes here:

This PR updates the documentation for the UPC++ installations at NERSC.

Submitters and reviewers please make sure

  • [N/A] internal links are relative and to the .md file
  • avoid links named "here"
  • [N/A] alt-text for images is descriptive
  • doesn't duplicate existing content
  • spell check
  • renders correctly
  • does not include any identifiable information (real usernames, project names, etc)

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