markdown_preview: Improve heading spacing (#59291)

Follow up to https://github.com/zed-industries/zed/pull/58465 adding
more breathing room between headings _and_ making inline code links use
the `accent` text color.

Release Notes:

- N/A
29b90dc4e8Danilo Leal committed on 6/14/2026, 12:11:15 PM· committed by GitHubparentc7987fa
1 file changedLine totals unavailable