Use acronyms consistently

Why is this change being made?

Updating the "Applicant Tracking System" and "Candidate Experience Specialist" instances to introduce the acronym on the initial use per the Editorial team's acronym guides. Currently, the document has mixed formatting.

When using acryonyms or initialisms, ensure that your first mention uses the full term and includes the shortened version in parentheses directly afterwards. From then on you may use the acronym or initialism instead.

If the acronyms aren't clear throughout the document, I'd recommend spelling out each instance.

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 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 in 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 relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)