Redesign README with hero section, visual tracks, and full course overview

[bdorlus]

Summary

Full README overhaul to make the repository's first impression reflect the quality and ambition of the course itself.

What changed

• Hero section — centered logo, bold tagline, stat pills (276 lessons, 31 chapters), live course site badge, GitHub Stars/License badges, and the operator loop SVG graphic — all above the fold
• "For the culture" mission statement — speaks directly to career switchers, self-taught learners, bootcamp grads, and anyone who's felt gated out of Linux knowledge
• Audience matrix table — 5 learner profiles (beginner, help desk, software engineer, cloud/DevOps/SRE, cybersecurity) each mapped to a clear value proposition
• Four-track ASCII learning map — visual block showing Foundations → Services → Operations Depth → Professional Practice with chapter ranges
• Collapsible 31-chapter table — all chapters with key skills, collapsed by default to keep the README scannable
• Time commitment table — per-lesson, per-chapter, full course, and capstone estimates
• Three quick-start options — web (zero setup), local make serve, static build
• Lesson structure breakdown — the 5 questions every lesson answers
• Capstone project summary — what it asks learners to build and prove
• Lab safety callout — reinforces the VM-first habit
• Contribute & community section — issue, PR, star, and share CTAs
• Closing social badges — star and fork shields with humanizing sign-off

Accessibility improvements (also on this branch)

• Glossary expanded from 12 → 65 terms covering all major course concepts
• Course overview updated with time estimates, operator loop diagram, safety boundaries, and chapter workflow guide
• Landing page (docs/index.md) updated with audience-based "Where to begin" quick-start cards
• mkdocs.yml updated with instant navigation, progress bar, breadcrumb path, and shareable search links

Test plan

• View the README on GitHub — verify hero renders, badges load, collapsible chapter list works
• Confirm mkdocs build --strict still passes
• Check that all links in the README resolve (course site, syllabus, glossary, capstone, lab setup, issues, PRs)

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