docs: Batch 1 — create automation hub + add Related sections

New page: docs/automation/index.md — single entry point for all automation
mechanisms (heartbeat, cron, tasks, hooks, standing-orders, webhooks) with
a decision flowchart and comparison table.

Add "Related" sections to 5 high-traffic pages that were dead ends:
- gateway/heartbeat.md → links to tasks, cron-vs-heartbeat, timezone, troubleshooting
- concepts/session.md → links to multi-agent, tasks, channel-routing
- concepts/multi-agent.md → links to channel-routing, subagents, ACP, presence, session
- concepts/agent-loop.md → links to tools, hooks, compaction, exec-approvals, thinking
- concepts/timezone.md → links to heartbeat, cron-jobs, date-time

Add automation/index to Mintlify nav as first item in Automation group.

docs/concepts/session.md

@@ -111,3 +111,6 @@ Preview with `openclaw sessions cleanup --dry-run`.
 - [Session Tools](/concepts/session-tool) -- agent tools for cross-session work
 - [Session Management Deep Dive](/reference/session-management-compaction) --
   store schema, transcripts, send policy, origin metadata, and advanced config
+- [Multi-Agent](/concepts/multi-agent) — routing and session isolation across agents
+- [Background Tasks](/automation/tasks) — how detached work creates task records with session references
+- [Channel Routing](/channels/channel-routing) — how inbound messages are routed to sessions