Notification service docs: Add the tutorial on how to create the slack workspace (#5939)

* add tutorial

* create workspace docs

* Edit img

Author: ctmphuongg

code-samples/eventing/bookstore-sample-app/create-workspace.md

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+# Notification Service: Creating a Slack Workspace
+
+## Create a Slack Workspace
+
+To create a Slack workspace, follow these steps:
+
+- Visit [https://slack.com/get-started#/createnew](https://slack.com/get-started#/createnew).
+- Enter your email address and click Continue, or continue with Apple or Google.
+- Check your email for a confirmation code.
+- Enter the code on the Slack website.
+- Click "Create a Workspace" and follow the prompts.
+- For detailed instructions for androids and , refer to [Create a workspace](https://slack.com/help/articles/206845317-Create-a-Slack-workspace).
+
+## Adding Channels to Your Workspace
+
+After creating your Slack workspace, you can add channels by following these steps:
+
+- Click the '+' icon next to 'Channels' on the sidebar of your Slack workspace.
+- Choose "Create a channel" and specify the channel name and purpose.
+- For enterprise grid organizations, refer to [Slack's help article](https://slack.com/help/articles/115001399587-Add-a-channel-to-multiple-workspaces-in-your-Enterprise-Grid-organization) for instructions.
+
+## Setting Up a Webhook for Notifications
+
+To set up a webhook for sending notifications to your Slack workspace:
+
+- Create a [Slack app](https://api.slack.com/apps/new), following the instructions by signing in, adding name and workspace to put the app in.
+- After this, you'll be redirected to the setting pages of your new app. Choose **Incoming Webhook** and toggle **Activate Webhook**.
+- Scroll down to click the option **Add New Webhook to Workspace**.
+  ![Alt text](./screenshots/select-channel.png)
+- The page will give you a link associated with the app you created. This link will be where API requests are sent to send messages to your Slack workspace.
+  ![Alt text](./screenshots/webhooklink.png)
+- To test if the webhook works as needed, you can try sending an API request to this link to see if the message goes through. I used Postman to test this. Remember that this is an application/json file.
+  ![Alt text](./screenshots/Postman.png)
+- Afterwards, the text channel should have a new message
+  ![Alt text](./screenshots/result.png)
+- Go to [Slack's webhook documentation](https://api.slack.com/messaging/webhooks) to learn more about setting up webhooks.