docs: add sandboxing page and cross-links

docs/install/docker.md

@@ -13,6 +13,8 @@ This guide covers:
 - Containerized Gateway (full Clawdbot in Docker)
 - Per-session Agent Sandbox (host gateway + Docker-isolated agent tools)
 
+Sandboxing details: [Sandboxing](/gateway/sandboxing)
+
 ## Requirements
 
 - Docker Desktop (or Docker Engine) + Docker Compose v2
@@ -92,6 +94,8 @@ pnpm test:docker:qr
 
 ## Agent Sandbox (host gateway + Docker tools)
 
+Deep dive: [Sandboxing](/gateway/sandboxing)
+
 ### What it does
 
 When `agent.sandbox` is enabled, **non-main sessions** run tools inside a Docker