Remove the text stroke on sidebar

[fern-support]

Overview

In response to this message - by default, Fern uses stroke rather than bolding to show the active tab in the left hand navigation. See this blog post:

Bold text is slightly wider than regular text, which caused some layout shifts in Fern Docs. This was most noticeable in header tabs: when switching tabs, the active tab's label would become bold and push neighboring tabs left or right. Similarly, top-level sidebar sections had no spacing between them, making them feel cramped.

This PR removes the stroke in replace of actual bold text for highlighted links in the left hand navigation

[github-actions]

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