Renumbered curriculum + topic-prefixed audio scheme

Enhance docs with GitHub accessibility guide content and rebuild HTML

- Ch02: Add official repos screen reader guide reference, landmark names tip,
  enhanced Watch section with Custom notification option
- Ch04: Add official issues screen reader guide reference, NVDA+F7 orientation
  tip, filter state caveat note
- Ch06: Add official PRs screen reader guide reference, Copilot actions button
  tip, Resolve Conversations section, Checks Tab section, enhanced diff
  navigation with Ctrl+Alt+Arrow keys, delete branch tip after merge
- Ch13: Add 3 official guide references (Copilot, custom instructions, custom
  agents), Shift+Alt+F1 screen reader mode toggle, Built-in Actions via
  Command Palette section, accessibility resources (A11y LLM Eval, Beast Mode
  Prompt, Markdown A11y Guidelines), video tutorial links for screen readers
- Rebuild all HTML from updated markdown (230 files)