Child topic page migration - what is developer first security

Migration of https://about.gitlab.com/topics/application-security/what-is-developer-first-security.index.html to new child topic template with Netlify CMS.

Related gitlab-com/marketing/inbound-marketing&285 (closed)

Review app link: https://im-285-what-is-developer-first-security.about.gitlab-review.app/topics/application-security/what-is-developer-first-security/

Why is this change being made?

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)