docs: clear, grouped prerequisites in the handbook and both READMEs

[project-delphi]

Closes #51.

What changed

The prerequisites were a single flat list that mixed do this beforehand with have this on the day, and left the required pre-work as the last bullet. They are now three labelled groups, every item linked:

• Do before the session — the book chapter, linear-algebra-deep-learning (still marked required), and running notebook 00 in Colab.
• Recommended — 3Blue1Brown's Essence of linear algebra, 16 videos, ~3 h. New workshop.prereq_video_url in _variables.yml, so both landing pages read it from the one source.
• Have ready on the day — a Google account (→ Colab), Discord, and a phone or second tab for Kahoot.

Four surfaces carry the same list: index.qmd, es/index.qmd, both halves of README.md, and the handbook (which hard-codes its links, as it already did, and speaks in Blocks rather than section numbers).

Discord is deliberately prose-only — "the invite comes from the facilitator". There is no public invite in the repo, and a placeholder URL is worse than none.

On the diff size

Two things beyond the prose:

1. docs/notebooks/ had gone stale against notebooks/ again — all twelve. Re-rendering syncs them, which is most of the line count. This is exactly the failure CLAUDE.md documents under Publishing: the regenerate gate never looks in docs/, and compare_render.py walks only *.html, so nothing catches it.
2. The Bootstrap and syntax-highlighting CSS get new content-hashed names — the macOS/ubuntu SCSS difference the same section describes. compare_render.py normalizes hashed asset names, so the publish gate is unaffected.

Verified

• gen_tables.py and gen_notebooks.py rerun with no tree drift.
• quarto render with the pinned 1.6.40.
• check_links.py: all eight failable checks pass (688 internal links, both landing pages, the 195-minute clock). Check 9 still prints the pre-existing Kahoot TODO, which is not a failure.

🤖 Generated with Claude Code

https://claude.ai/code/session_0125vs3ARnz2pLp8D5vQ6iKJ