site: add Deep dive links per architecture layer

Each arch-layer card now links to a high-quality canonical post on
terraphim.ai. Layer 2 points to the new why-graph-embeddings-matter
post; other layers point to the closest existing post and will be
backfilled as the daily-cadence campaign publishes them.

Refs #580 #583

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: AlexMikhalev