doc: fix left nav color contrast
I believe this is the last change to make in the docs for color contrast accessibility per WCAG AA.
Currently, the current item in the nav is #43853d on a #333333 background for a color contrast of 2.8:1, failing WCAG AA. This swaps in removes a confusing bottom border on the item, replacing it with bolding to preserve the item's highlighting.
Checklist
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