Remove compiled stylesheet subdirectories

Why is this change being made?

This MR moves the one stylesheet that is compiled to a subdirectory in the /stylesheets/devops-tools directory to just /stylesheets. I am doing this for consistency - so all of our compiled css is located in /stylesheets. This will also facilitate and simplify the webpack external asset pipeline coming soon.

Specifically, this MR moves source/stylesheets/devops-tools/extra-content.css.scss to source/stylesheets/extra-content-devops-tools.css.scss. This MR also updates all the files that reference the compiled stylsheet with the new build destination and filename.

Author Checklist

• Provided a concise title for the MR
• [s] Added a description to this MR explaining the reasons for the proposed change, per [say-why-not-just-what][transparency]
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately 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][mr-buddies-slack].
  • 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.

Related #5323 (closed)