Fix broken Linux command-line guide link

[dlanov]

Summary

This PR fixes one broken external link from issue #4209.

The old beginner Linux command-line guide link in Creating-A-Workspace.rst no longer works reliably, so this PR replaces it with Ubuntu’s beginner command-line tutorial.

Changes

• Replaced the broken SFU Linux command-line guide link.
• Kept the change limited to one beginner tutorial prerequisite link.

Testing

I ran:

• make html
• make lint
• make spellcheck
• curl -I https://ubuntu.com/tutorials/command-line-for-beginners

The replacement link returned HTTP/2 200.

make linkcheck may still report unrelated broken links tracked by #4209.

Part of #4209.

AI assistance disclosure

I used Claude Code to help locate and edit the link, then manually reviewed the diff and test results before submitting.

[dlanov]

I updated the replacement link to use the newer Ubuntu Desktop documentation URL.

I re-ran:

• make html
• make lint
• make spellcheck
• curl -I https://ubuntu.com/desktop/docs/en/latest/tutorial/the-linux-command-line-for-beginners/