docs: continue channel onboarding/config naming cleanup (#22533)

* docs: fix thinking link and add reasoning anchor reference

* docs(channels): rename LINE setup heading to onboarding

* docs(channels): normalize Nextcloud Talk onboarding headings

* docs(channels): use onboarding heading for Matrix setup

* docs(channels): standardize Discord onboarding heading

* docs(channels): standardize Telegram onboarding heading

* docs(channels): standardize WhatsApp onboarding heading

* docs(channels): rename iMessage onboarding and configuration sections

* docs(channels): rename Slack onboarding and configuration sections

* docs(channels): rename Signal onboarding heading

docs/channels/imessage.md

@@ -28,7 +28,7 @@ Status: legacy external CLI integration. Gateway spawns `imsg rpc` and communica
   </Card>
 </CardGroup>
 
-## Quick setup
+## Onboarding
 
 <Tabs>
   <Tab title="Local Mac (fast path)">
@@ -358,7 +358,7 @@ imsg send <handle> "test"
   </Accordion>
 </AccordionGroup>
 
-## Configuration reference pointers
+## Configuration
 
 - [Configuration reference - iMessage](/gateway/configuration-reference#imessage)
 - [Gateway configuration](/gateway/configuration)

docs/channels/signal.md

@@ -17,7 +17,7 @@ Status: external CLI integration. Gateway talks to `signal-cli` over HTTP JSON-R
 - A phone number that can receive one verification SMS (for SMS registration path).
 - Browser access for Signal captcha (`signalcaptchas.org`) during registration.
 
-## Quick setup (beginner)
+## Onboarding
 
 1. Use a **separate Signal number** for the bot (recommended).
 2. Install `signal-cli` (Java required if you use the JVM build).

docs/channels/slack.md

@@ -21,7 +21,7 @@ Status: production-ready for DMs + channels via Slack app integrations. Default
   </Card>
 </CardGroup>
 
-## Quick setup
+## Onboarding
 
 <Tabs>
   <Tab title="Socket Mode (default)">
@@ -487,7 +487,7 @@ channels:
 - Media and non-text payloads fall back to normal delivery.
 - If streaming fails mid-reply, OpenClaw falls back to normal delivery for remaining payloads.
 
-## Configuration reference pointers
+## Configuration
 
 Primary reference: