fix: DocSearch covered by nav because of z-index

[gabrielkoo]

Resolves #159

Background

The current issue with DocSearch modal being covered by nav bar arose as a side effect of the fix in #112 and #113.

In #112, an attempt was made to avoid the Youtube videos' "total time" to be floating on the top nav by making the nav's z-index to be 9999. It did solve the issue but it caused #159 too.

In #113 the issue was fully fixed by adjusting the z-index of the "total time" element itself, so the fix in #112 was no longer necessary.

Actually, the z-index for the top nav is already managed by z-index: var(--ifm-z-index-fixed); so manually specifying a value while not using CSS variables might easily break other styles.

Screenshots

Before	After

Tests

Tested the following functionalities are working on the preview site: https://docs-git-fork-gabrielkoo-fix-modal-covered-by-nav-dailydotdev.vercel.app/video-tutorials

• [x] DocSearch Modal is no longer blocked by top nav
• [x] Youtube video "total time" is not floating on top nav

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Name	Status	Preview	Updated
docs	✅ Ready (Inspect)	Visit Preview	Oct 10, 2022 at 2:53AM (UTC)

[gabrielkoo]

@GrahamTheDev Could you help reviewing this pull request? This feature should be quite troublesome for some users 😢.

[FrancescoXX]

thanks