Create a new documentation by clicking on the New button again and selecting API Documentation.
There are two different ways tho create an API Documentation:
Select the tab "Create a new API". By choosing this option, Postman will create your API requests and document them. Remember to add a description on the requests.
Type the general information of your API in the description box, you can use Markdown (.md) to format the text.
Click the Create button and then Close.
Select the tab "Use collection from this workspace". Your current collections will be shown in the screen. Select the one you want to document.
Type the general information of your API in the description box, you can use Markdown (.md) to format the text.
Click the Create button and then Close.
To access your documentation, find in the sidebar the collection, hover your mouse over it and then press the > button on the right.
Click "View in web" to view your complete documentation: