docs(architecture): consolidate lab architecture docs

[jmgilman]

Summary

• replace the stale architecture overview with a focused architecture doc set for the IncusOS/Incus/Talos baseline
• fold stable design decisions for bootstrap, networking, GitOps, secrets, Keycloak, and recovery into architecture docs
• remove promoted proposed-design pages so the docs no longer present parallel baselines

Validation

• git diff --check
• moon ci --summary minimal
• moon run docs:check

Notes

• Prototype-dependent areas are called out in the architecture docs instead of being over-specified.
• Runbook pages such as RouterOS ACME remain separate and linked from architecture where relevant.