docs: add learning progress checklist to README

[aryansk]

Summary

Fixes #1026.

Adds an accessible, copyable learning-progress checklist to README.md using the primer's existing sections. The checklist is grouped into foundations, core building blocks, data/communication/security, and practice so readers have a suggested sequence without introducing new content or external dependencies.

The note explains how readers can copy the checklist into a personal note or fork to preserve their own progress; the canonical README remains unchanged for everyone.

Validation

• pandoc --from=gfm --to=plain README.md -o /dev/null
• git diff --check
• Verified all 17 checklist anchors resolve to headings in README.md