Add ARPA acronym for searchability and reference to Sales Handbook

Why is this change being made?

Add ARPA acronym for searchability and reference to Sales Handbook

"ARPA" was used in a finance call, though there is no instance of "ARPA" within the GitLab Handbook.

Average Revenue Per Account (ARPA) is sometimes called Average Revenue Per User (ARPU) or Average Revenue Per Customer (ARPC).

This proposal adds the acronym to the existing Definition of ARPU on the page, such that anyone searching the handbook for ARPA instead of ARPU will find the right page.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the change relates to any part of the project other than updates to content and/or data files (e.g. team.yml) please make sure to ping @gl-static-site-editor in a comment for a review and merge.
  • If the DRI for the page/s being updated isn't immediate clear, then assign it to your manager.
  • 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

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