docs: add docs/README.md index warning about historical design-docs

[MarcoPorcellato]

Context

docs/design-docs/ contains rich historical blueprints that can mislead newcomers. A documentation index (docs/README.md) was added to distinguish active vs historical docs — verify completeness and cross-links.

Objectives

• Review docs/README.md: active vs historical table is accurate
• Link to ARCHITECTURE.md, logseq_ast_primer.md, GOOD_FIRST_ISSUES.md
• Add one-line pointer in CONTRIBUTING.md Documentation section (if missing)
• Optionally add a one-line banner at the top of docs/design-docs/README.md (new stub) pointing to docs/README.md

Files

• docs/README.md
• CONTRIBUTING.md
• Optional: docs/design-docs/README.md

Getting started

1. Read docs/design-docs/CODE_SCAFFOLD.md banner text for tone
2. Walk the docs/ tree and confirm the index matches reality

Out of scope

• Rewriting historical design docs

Definition of Done

• All links resolve
• Optional CHANGELOG.md ### Added for docs index

[PritishRaman]

Hi! I'd love to work on this good first issue. Could you please assign it to me?

[MarcoPorcellato]

Hi, thanks for the request and welcome to this repo!

You are now in charge of this issue.
Marco