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/docs.json

@@ -1114,6 +1114,7 @@
               {
                 "group": "Automation",
                 "pages": [
+                  "automation/index",
                   "automation/hooks",
                   "automation/standing-orders",
                   "automation/cron-jobs",