docs: more channel onboarding heading consistency (#22532)

* 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/discord.md

@@ -21,7 +21,7 @@ Status: ready for DMs and guild channels via the official Discord gateway.
   </Card>
 </CardGroup>
 
-## Quick setup
+## Onboarding
 
 You will need to create a new application with a bot, add the bot to your server, and pair it to OpenClaw. We recommend adding your bot to your own private server. If you don't have one yet, [create one first](https://support.discord.com/hc/en-us/articles/204849977-How-do-I-create-a-server) (choose **Create My Own > For me and my friends**).
 

docs/channels/telegram.md

@@ -21,7 +21,7 @@ Status: production-ready for bot DMs + groups via grammY. Long polling is the de
   </Card>
 </CardGroup>
 
-## Quick setup
+## Onboarding
 
 <Steps>
   <Step title="Create the bot token in BotFather">

docs/channels/whatsapp.md

@@ -21,7 +21,7 @@ Status: production-ready via WhatsApp Web (Baileys). Gateway owns linked session
   </Card>
 </CardGroup>
 
-## Quick setup
+## Onboarding
 
 <Steps>
   <Step title="Configure WhatsApp access policy">