Add account link instructions

What does this MR do and why?

This MR adds a minor change to the Change the linked account instructions. If no accounts are linked with the customer portal, the UI shows a different option to link the GitLab.com account. This change provides more clarity to new customers.

Screenshots or screen recordings

How to set up and validate locally

1. Create a new customer portal account: https://customers.gitlab.com.
2. Navigate to My Account and expand the Your GitLab.com account section.
3. Notice the options displayed for new customers.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.