Follow-up to `handbook.css.scss` refactor for blog `layout: default` use

After #6853 (closed) is complete, apply this follow-up.

• The issue is that the default template is used in the handbook, but also the blog. As such, the refactored styles into just a single handbook.css.scss file means the blog will get additional unused styles sent to the client.
• The handbook.js file is currently also needed for the security.haml's use of the toc and related CSS

*This bloated styling issue was the case prior to and the reason for the refactor to begin with. However, we can iterate and breakup styles even more granularly