Tracking update `mkbook` dependency once pulldown-cmark fix lands

[tanishiking]

#626 was necessary due to an issue in pulldown-cmark pulldown-cmark/pulldown-cmark#356 .

This issue is to track when mkbook updates its pulldown-cmark dependency to a version that includes the fix
rust-lang/mdBook#2387.

Once mkbook incorporates the fix, we should remove the workaround from our code.