moved shell extra lesson reference

[sstevens2]

fixes #1455

• Removed the reference to shell extra lesson in the prereqs
• Created Additional Resources page for learners and added text about shell extra lesson

Might recommend we open a new issue looking for additional resources in include on that page, it is a little sparse at present.

[github-actions]

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Rendered Changes

🔍 Inspect the changes: https://github.com/swcarpentry/shell-novice/compare/md-outputs..md-outputs-PR-1494

The following changes were observed in the rendered markdown documents:

 index.md           |  5 -----
 md5sum.txt         |  3 ++-
 resources.md (new) | 10 ++++++++++
 3 files changed, 12 insertions(+), 6 deletions(-)

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⏱️ Updated at 2025-06-18 15:50:49 +0000

[bkmgit]

Thanks.