docs: improve app creation for incoming webhook technique

📚 The setup steps of the incoming webhook technique require multiple steps on an adjacent UI. We might suggest creating an app with the following manifest:

🔗 https://docs.slack.dev/tools/slack-github-action/sending-data-slack-incoming-webhook#setup

{
  "_metadata": {
    "major_version": 1
  },
  "display_information": {
    "name": "GitHub Actions",
    "description": "Incoming webhook for GitHub Actions notifications"
  },
  "features": {
    "bot_user": {
      "display_name": "GitHub Actions"
    }
  },
  "oauth_config": {
    "scopes": {
      "bot": ["incoming-webhook"]
    }
  },
  "settings": {
    "org_deploy_enabled": true,
    "incoming_webhooks": {
      "incoming_webhooks_enabled": true
    }
  }
}

Open the linked incoming webhook setup section and compare its current app-creation steps with the proposed manifest. Update the documentation to suggest creating the app from that manifest, then verify the setup instructions and rendered documentation are clear and complete.