feat: build MUSA GitHub Pages knowledge base

[googs1025]

Summary

• turn the Pages root into a dual-entry MUSA knowledge hub with Week 1–6 and topic navigation
• add six weekly knowledge pages, the MPC/MPX/MP hierarchy article, curated code points and questions
• preserve the 150-question quiz at quiz.html and harden Pages/link validation with 38 regression tests
• refine the related learning-guide prose and commit the approved design/implementation plans

Test Plan

• node scripts/check-docs.test.js (38/38)
• node scripts/check-docs.js (150 questions, 9 pages, 136 local links)
• git diff --check origin/main...HEAD
• headless Chrome at 1440x900 and 375x812: search, details, Quiz controls, code/table scrolling, no horizontal page overflow

Deployment

The Pages workflow validates pull requests and deploys the docs/ artifact after merge to main.