Refine usage quota tips in pricing FAQ

Why is this change being made?

The pricing FAQ for paid/free explains the usage quota location, but it is a little hard to find in the UI. This MR adds a URL to the documentation, and also adds the detail to navigate into the Storage tab.

The answer to the question How can I view and manage my storage usage? duplicates the storage types but misses a few. I have removed the duplicate SSoT, and pointed to the question above.

I've also refined the wording of artifacts to Pipeline job artifacts - if you are not deeply familiar with GitLab, this additional info can help with faster analysis.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

---