docs: more channel onboarding naming cleanup (#22536)

* 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): standardize Nostr onboarding and configuration headings

* docs(channels): standardize Zalo onboarding and configuration headings

* docs(channels): standardize Twitch onboarding heading

* docs(channels): standardize Google Chat onboarding heading

* docs(channels): standardize Mattermost onboarding heading

* docs(channels): standardize Zalo Personal onboarding heading

docs/channels/googlechat.md

@@ -9,7 +9,7 @@ title: "Google Chat"
 
 Status: ready for DMs + spaces via Google Chat API webhooks (HTTP only).
 
-## Quick setup (beginner)
+## Onboarding
 
 1. Create a Google Cloud project and enable the **Google Chat API**.
    - Go to: [Google Chat API Credentials](https://console.cloud.google.com/apis/api/chat.googleapis.com/credentials)

docs/channels/mattermost.md

@@ -33,7 +33,7 @@ OpenClaw will offer the local install path automatically.
 
 Details: [Plugins](/tools/plugin)
 
-## Quick setup
+## Onboarding
 
 1. Install the Mattermost plugin.
 2. Create a Mattermost bot account and copy the **bot token**.

docs/channels/zalouser.md

@@ -27,7 +27,7 @@ The Gateway machine must have the `zca` binary available in `PATH`.
 - Verify: `zca --version`
 - If missing, install zca-cli (see `extensions/zalouser/README.md` or the upstream zca-cli docs).
 
-## Quick setup (beginner)
+## Onboarding
 
 1. Install the plugin (see above).
 2. Login (QR, on the Gateway machine):