docs: add SDK documentation as a peer section to Platform and API Reference

[ethanj]

Summary

Adds a dedicated SDK section to the docs site (docs.atomicmemory.ai) with equal navigational presence to Platform and API Reference. The SDK is framed as a platform utility agnostic to the memory backend — the pluggable MemoryProvider architecture is the headline, runtime capability inspection is the contract, and the site stays inside the public SDK surface (no internal consumption-architecture advice).

20 hand-authored pages, 8 mermaid diagrams (architecture + dataflow), and minimal cross-linking updates to existing Core pages.

What's in

• Landing: sdk/overview (with the deployment topologies diagram), sdk/quickstart
• Concepts (7): architecture, memory providers, capabilities, scopes and identity, storage adapters, embeddings, consent and gating
• Guides (7): atomicmemory backend, mem0 backend, writing a custom provider, browser helpers, Node.js / server-side, browser primitives, swapping backends
• API Reference (3 hand-written): reference overview, MemoryProvider contract, errors. Type-level signatures are deferred to the SDK's existing typedoc pipeline rather than hand-maintained here (prevents drift).
• Cookbook (1): loadKnowledgeBase

Diagrams (mermaid, via @docusaurus/theme-mermaid)

1. High-level SDK architecture (flowchart)
2. Provider architecture (classDiagram)
3. Ingest sequence
4. Search with domain gating — matches actual contract ({ results: [] } on blocked)
5. Package with extension probe — matches actual contract (getExtension<Packager>('package'))
6. Storage subsystem
7. Deployment topologies (four side-by-side shapes)
8. Embeddings pipeline

Cross-links (existing pages)

• introduction.md: replaces "coming soon" with links to /sdk/overview
• quickstart.md: adds a closing "Using the TypeScript SDK" section
• platform/providers.md: disambiguation callout (embedding/LLM providers ≠ memory providers)
• platform/scope.md: see-also pointer to the SDK's client-side scope page
• docs/api-reference/sdk/coming-soon.md removed; @docusaurus/plugin-client-redirects serves /api-reference/sdk/coming-soon → /sdk/overview (GitHub Pages compatible)

Config

• New deps: @docusaurus/theme-mermaid@3.10.0, @docusaurus/plugin-client-redirects@3.10.0
• docusaurus.config.ts: mermaid enabled, SDK navbar item inserted between Platform and API Reference, redirect plugin registered, footer updated
• sidebars.ts: new sdkSidebar; coming-soon entry stripped from apiReferenceSidebar

Open questions captured in the docs (not blockers)

• SDK publish status on npm — Quickstart uses npm install @atomicmemory/atomicmemory-sdk; verify before external users follow it
• Mem0Provider public-support stance — the Mem0 guide carries a support-status note
• Cross-repo typedoc pipeline for generated reference — deferred follow-up

Test plan

• npm run build passes with onBrokenLinks: 'throw'
• All 20 SDK pages render to build/sdk/**
• All 8 mermaid diagram sources compiled into JS bundles (spot-checked D3, D5, D7)
• Redirect stub at build/api-reference/sdk/coming-soon/index.html uses meta refresh → /sdk/overview
• Navbar order: Platform | SDK | API Reference | GitHub
• Visual smoke test on npm start: click through every SDK sidebar entry, confirm each of the 8 diagrams actually renders (mermaid hydrates client-side; worth eyeballing)
• Mobile responsive pass on mermaid diagrams
• GitHub Pages deploy confirmed on docs.atomicmemory.ai

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