tailwind config for highlighting back links in source snippets

Summary:
most of the docs pre tag anchors are these dark-mode source snippets.
Ensures that these anchors are visible inside of the dark-background pre tags.

Test Plan: inspection, clicked around, checked api docs, general docs

Reviewers: yuhan

Reviewed By: yuhan

Differential Revision: https://dagster.phacility.com/D9035