Improve README note navigator

[xy-241]

Summary

• add a quick-jump index for the Notes Navigator section
• group each topic collection into collapsible details for easier scanning
• tidy the lists so anchors are linkable and consistent

Testing

• not run

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https://chatgpt.com/codex/tasks/task_e_690600c5fa388332a03f91e6d065793f

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[chatgpt-codex-connector]

Close each notes section’s <details> element

The Notes Navigator converts every topic into a <details> block but only the final Networking section is closed. Starting at the System Design block, each subsequent <details> is opened without a matching </details> until line 173, so the sections become nested rather than siblings and toggling any earlier summary collapses or expands everything that follows. Adding a closing tag after each section restores independent expand/collapse behaviour.

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[deepsource-io]

Here's the code health analysis summary for commits 558529c..f5d2ec3. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
JavaScript	✅ Success		View Check ↗

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