Add beginner course orientation and contextual checkpoints

[Doppp]

Summary

• add a dedicated course overview and a new beginner-first “How machines learn” lesson before gradient descent
• place Guided predict, experiment, apply, and reflect controls beside the lesson content they govern
• add the one-parameter learning-loop explorable and update the reusable course template
• version the course as 0.5.0-onboarding.1 and update the PRD, architecture, authoring guide, implementation plan/status, roadmap, README, and changelog

Verification

• pnpm check
• pnpm build
• pnpm course:test (18 suites / 57 tests; 14 starter/reference contracts)
• pnpm test:browser (16 tests)
• pnpm site:test (2 tests)

All checks were run from a clean Node 24.19.0 / pnpm 11.19.0 worktree. The four paused learner exercise edits in the primary worktree are intentionally excluded.