Resolve "Markdown header `code` does not have the correct font-size"

What does this MR do?

set $code_font_size to 90% - used in the similarly-named bootstrap variable: https://github.com/twbs/bootstrap/blob/v4-dev/scss/_variables.scss#L936

Are there points in the code the reviewer needs to double check?

maybe wikis or builds or somewhere I've forgotten that we explicitly wanted wanted code to be 12px... However 87.5% of our base font size (14px) is 12.25 it is the same anyway. There are cases where that would break.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them

What are the relevant issue numbers?

Closes #48549 (closed)