More padding around nav bar links

The nav bar links are spaced like they're all part of the same paragraph. Given some of their long and title-cased names, I honestly though some of them were wrapping around to two lines, but actually it's two separate links.

Adding a little more vertical whitespace would distinguish each link, and give them larger click/touch targets that I think would be more convenient. Here's how it looks with `.vp-navbar-dropdown-item a { line-height: 2em !important; }`:

找到控制 `.vp-navbar-dropdown-item a` 的樣式表或主題項目，然後檢查現有的 navbar 連結間距。套用要求的額外垂直留白，並驗證下拉式連結在一般文字寬度和文字換行寬度下都具有視覺區別，且擁有更大的點擊／觸控目標。

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