Add slack app manifest and update the configuration guide

[wyangsun]

This pull request adds documentation and configuration to support creating and configuring a Slack app for the project. The most important changes include the addition of a Slack app manifest file and updates to the setup instructions to clarify manual steps required after using the manifest.

Slack App Manifest and Configuration:

• Added a new slack_app_manifest.json file that defines the Slack app’s name, description, bot user settings, OAuth scopes, event subscriptions, and other configuration options. This manifest simplifies initial app creation in Slack.

Documentation Updates:

• Updated docs/configuration.md with a new section explaining how to create a Slack app using the manifest, and clarified which settings must still be configured manually (e.g., creating tokens and enabling direct messages for the bot).