docs: comprehensive OpenClaw migration guide (#3900)

New standalone guide at guides/migrate-from-openclaw.md with:
- Complete config key mapping tables for every category
- Agent behavior mappings (thinkingDefault → reasoning_effort, etc.)
- Session reset policy mapping (session.reset vs resetTriggers)
- TTS dual-source explanation (messages.tts.providers + talk config)
- MCP server field-by-field mapping
- Messaging platform table with exact config paths and env vars
- API key resolution: 3 sources, priority order, supported targets
- SecretRef handling: plain strings, env templates, SecretRef objects
- Post-migration checklist (6 steps)
- Troubleshooting section
- Complete archived items table with recreation guidance

CLI commands reference condensed to summary + link to full guide.
Added to sidebar under Guides & Tutorials.

website/sidebars.ts

@@ -26,6 +26,7 @@ const sidebars: SidebarsConfig = {
         'guides/use-mcp-with-hermes',
         'guides/use-soul-with-hermes',
         'guides/use-voice-mode-with-hermes',
+        'guides/migrate-from-openclaw',
       ],
     },
     {