S23.16: Core page, architecture overview diagram, and PlusTcpResolver accuracy fix #627

Part of the API-reference doorway (follows #624 / S23.14).

Scope

• Core doorway page (docs/core/index.md) and its nav entry — the facade, what Core ships, cross-cutting headers, and requirements.
• Architecture overview diagram — a layered touch-points diagram (Core as a boundary; the roles that reach the platform cross it through a thin adapter to the third party you supply). Generated by scripts/gen_architecture_overview.py; the post-it README notes the generated-diagram exception. Supersedes the WIP radial wheel (added then removed on the branch).
• PlusTcpResolver accuracy fix — corrects the docs (and CLAUDE.md) that described PlusTcpResolver as a hardcoded-IPv4 resolver; it is a DNS resolver over FreeRTOS_getaddrinfo.

Acceptance

• Core page renders in the site and its diagram links resolve (facade → header page, roles → struct pages, platform-adapter band → Platforms).
• docs-build is green.

Parent epic: #246