Navbar on top of Vue mastery banner on mobile

[clicktodev]

Only happens when I scroll

[xfq]

It's probably because the z-order of the VPNav component is larger than the Vue Mastery banner.

[clicktodev]

Will attempt a PR.

[xfq]

FYI - the banner component is in https://github.com/vuejs/docs/blob/main/.vitepress/theme/components/VueMasteryBanner.vue

[clicktodev]

This pr should fix it.