docs: rewrite appendix-e as comprehensive Markdown and GFM tutorial

- Expanded from 490 lines/11 sections to 1,561 lines/32 sections across 3 parts
- Part 1: Markdown Foundations (sections 1-16) - paragraphs, headings, emphasis,
  lists, links, images, blockquotes, code blocks, tables, and more
- Part 2: GitHub Flavored Markdown (sections 17-27) - alerts, collapsible sections,
  task lists, syntax highlighting, Mermaid, math/LaTeX, footnotes, autolinked refs
- Part 3: Putting It All Together (sections 28-32) - screen reader behavior summary,
  accessible authoring checklist, common mistakes, guided exercise, quick reference
- Every section includes raw Markdown, rendered output, and screen reader notes
- Updated course-guide.md description for Appendix E
- Rebuilt HTML for GitHub Pages site

Author: GitHub Workshop Bot