Document nav logo CSS specificity

[hamedrabah]

Documentation changes

Clarifies that custom logo sizing should target img.nav-logo and includes a CSS example that overrides the default Tailwind height utility while preserving the logo aspect ratio.

Closes #6242

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For Reviewers

• The selector matches Mintlify template usage.
• The example addresses the documented specificity conflict.
• The change is limited to the affected navigation hook.

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Note

Low Risk
Documentation-only change to custom CSS guidance; no runtime or product code affected.

Overview
Updates the Topbar navigation element-selector docs for nav-logo so custom sizing guidance matches how the logo is rendered in the template.

The nav-logo entry now states it applies to the logo image, explains that default size utilities need img.nav-logo with !important, and adds a short CSS example (height: 3.75rem, width: auto) for overriding Tailwind height while keeping aspect ratio. Closes #6242.

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[hamedrabah]

Ready for review when a maintainer has a chance. This is a small docs-only correction for #6242, and the automated check is passing. Thank you!