Add next-class-day-plan shortcode and wire to ITP index (fixes #1119)

[JohnnyRobs19]

What does this change?

• Adds a shortcode (common-theme/layouts/shortcodes/next-class-day-plan.html) that computes the next likely class day-plan and renders a direct link.
• Wires the shortcode into org-cyf-itp/content/_index.md so the module index shows a single "Next class" link.
• Heuristic used: next Saturday -> match course start month -> compute week/sprint -> index into the "scheduled" menu. Falls back when no data available.

Common Content?

• Block/s

Common Theme?

• Yes

Issue number: #1119

Org Content?

Module	Sprint	Page Type	Block Type
org-cyf-itp	N/A	Module index page (_index.md)	N/A

Checklist

• I have read the contributing guidelines
• I have checked spelling/grammar
• I have previewed my changes locally
• I have run my code to check it works

Who needs to know about this?

Please review: @SallyMcGrath

Before / After (artifacts)

Before

• Full-page screenshot (current UI)
• Editor screenshot or paste of org-cyf-itp/content/_index.md (pre-change)
• Small reproduction command snippet:

cd ~/code/curriculum/org-cyf-itp
sed -n '1,200p' content/_index.md > before__index.md
hugo server -D --bind=0.0.0.0 --port=1313
# open http://localhost:1313

After

• Full-page screenshot showing the rendered Next class link
• The new shortcode file: common-theme/layouts/shortcodes/next-class-day-plan.html
• The updated file: org-cyf-itp/content/_index.md
• Preview commands:

cd ~/code/curriculum
git checkout -b updated-link-1119
# add the new and modified files
git add common-theme/layouts/shortcodes/next-class-day-plan.html org-cyf-itp/content/_index.md
git commit -m "feat(next-class): add next-class-day-plan shortcode and wire to ITP index (#1119)"
hugo server -D --bind=0.0.0.0 --port=1313
# open http://localhost:1313

Notes

• The shortcode is referenced in org-cyf-itp/content/_index.md using {{< next-class-day-plan >}}.
• If a site build fails with an error like failed to render shortcode "next-class-day-plan", ensure the shortcode file path is exactly common-theme/layouts/shortcodes/next-class-day-plan.html and that the partials it calls (if any) exist at the expected paths.

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[illicitonion]

Thanks for putting this together! It generally looks reasonable, but one thing I'm not sure about is the scheduled menu - what are you expecting to be added to it?

As far as I can tell, this pull request doesn't create a menu called scheduled - what's meant to be in it? It looks like it's meant to be each sprint?

We already have a menu called "syllabus" which contains each module, and then a menu_level called "module" which contains each sprint within a module - could we use these existing menus, rather than having to introduce another one?