Docs: add an internal index for project docs

[nerdchanii]

Summary

• add a docs/project/README.md index that explains the purpose of each maintainer-facing project note
• clarify the main docs structure page so maintainers have one entry point into internal docs
• record where a future SPEC would become the source of truth without introducing a full SPEC/ADR system

Type

• API
• Runtime
• Testing
• Docs
• Tooling
• Packaging/release

Priority

• P0 - Critical
• P1 - High
• P2 - Medium
• P3 - Low
• P4 - Backlog

Verification

• Tests added or updated
• Typecheck passes
• Build passes
• Docs/examples updated if needed

Release Intent

• Changeset added
• No release needed (docs/project-only or tooling-only and does not affect consumers)

Notes

Related issue: Closes #120

Verification:

• npm run docs:build
• npm run lint

[nerdchanii]

@codex review

Context:

• Issue: Docs: Audit and reorganize internal project docs #120, milestone: post-v1 stabilization

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. More of your lovely PRs please.

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