Restore diagrams and Fumadocs migration parity

[charitra-prem]

What

• restore the memory-scope diagram that was left as an editorial placeholder
• restore the knowledge-graph diagram from the pre-migration documentation
• add accessible, responsive diagram components that follow the existing Fumadocs theme
• restore the light default theme, social links, release-notes icon, and missing GitHub card mark

Audit

• all 29 migrated documentation routes are present
• all internal links and anchors resolve
• every referenced local image exists and has alt text
• removed connector and MCP screenshots were confirmed as later product changes, not migration losses

Verification

• npm run build
• static browser review at desktop width