If you are setting up your own universal forwarder (or heavy forwarder for that matter) you will need your Splunk Cloud instance credentials (as you noted). This is available as an app that you install into your universal forwarder and is downloadable from your Splunk Cloud instance.
In the docs.splunk.com URL you posted, steps 1-4:
1) Log into your Splunk Cloud deployment
2) Download the Splunk Universal Forwarder for Linux.
3) Install the Splunk Universal Forwarder for Linux.
4) Download and install the universal forwarder credentials.
The universal forwarder credentials are within your Splunk Cloud instance.
Log into your Splunk Cloud instance (as an admin role account). On the main page on the left apps browser list there will be a 'Universal Forwarder' app. Click on that and the app menu below will be presented:
To set up the Universal Forwarder:
Download the Splunk universal forwarder.
Splunk Downloads web page
Install the universal forwarder on one or more machines in your network.
Download your customized universal forwarder credentials package.
**Download Universal Forwarder Credentials**
Install the universal forwarder credentials package on each universal forwarder in your network.
Configure your universal forwarders to send data to the Splunk platform.
Configure data inputs
The Download Credentials button is where you need to grab your Splunk Cloud credentials (app).