Add CHANGELOG entry for the nav bar overlap fix

[e-koch]

Summary

• Adds the missing CHANGELOG.md entry for Fix nav bar overlapping the top of rendered notebook pages #56 under Unreleased → Fixed. It notes that the nav bar padding from Add gap below nav bar on rendered notebook pages #53 never took effect because nbconvert's .jp-Notebook styles overrode it.
• The Add gap below nav bar on rendered notebook pages #53 entry under Added is left as-is. The new line explains how it relates to Fix nav bar overlapping the top of rendered notebook pages #56.

Test plan

• Docs-only change. Checked that the entry follows the existing format (one line, PR number at the end) and oldest-first order within the section.

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