Document non-billable usage on GitLab Credits dashboard

What does this MR do?

Updates the GitLab Credits dashboard docs for the non-billable usage now shown on the Total credit consumption chart:

• Removes the outdated note at the top of the "In GitLab" section, and says the Organization usage summary cards show billed PLUS non-billed usage.
• Says the chart is a stacked bar chart of billed and non-billable usage.
• Adds a Non-billable usage subsection that explains the Billed and Non-billable totals and lists each non-billable usage series: Non-billable usage (Promotional), Non-billable usage (Beta), and Non-billable usage (Other). Usage with no recorded reason is included in Other. The Internal series is not documented because only GitLab team members see it.

Note: The UI change is in !260225 (merged) & !260725 (merged).

Related issues

• Resolves https://gitlab.com/gitlab-org/gitlab/-/work_items/632422
• Epic: https://gitlab.com/groups/gitlab-org/-/work_items/23876
• Frontend MR: !260225 (merged) & !260725 (merged)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
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