Add Appendix AC (GitHub CLI), AD (Git Security), AE (GitHub Social)

Appendix AC: GitHub CLI Reference
- 13 sections: install/auth, repos, issues, PRs, releases, search,
  labels/milestones, JSON/jq output, aliases, extensions, Copilot CLI,
  screen reader tips, quick reference card

Appendix AD: Git Security for Contributors
- 9 sections: why it matters, .gitignore deep dive, env variables,
  review before commit, pre-commit hooks, accidental secret recovery,
  GitHub push protection, secure credential storage, checklist

Appendix AE: GitHub Social — Stars, Following, and Finding Your Community
- 12 sections: stars, watching, following people, home feed, Explore,
  Trending, Topics, Lists, finding accessible projects, building
  your presence, CLI commands, screen reader navigation guide

docs/course-guide.md: Added AC, AD under Git Reference; added new
  'GitHub Community and Social' section with AE

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: taylorarndt