Include missing step in CI/CD group purchase flow

What does this MR do and why?

Adds a step in our documentation to guide the user to select the "Pipelines" tab in order to proceed with purchasing additional CI/CD minutes for their group.

Screenshots or screen recordings

Currently the experience drops the user into the "Seats" section of "Usage Quotas":

But the user needs to manually navigate to "Pipelines" in order to see the "Buy Additional Minutes" button. This is not reflected in our documentation.

How to set up and validate locally

1. On the top bar of GitLab, select Menu > Groups and find your group.
2. On the left sidebar, select Settings > Usage Quotas.
3. (You must then click "Pipelines" in order to proceed with the next step)
4. Select Buy additional minutes.
5. Complete the details of the transaction.

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.