navbar shouldn't contain the navbar mobile sidebar

[slorber]

We should be able to set different stacking contexts (ie z-index layers) for the navbar and the mobile sidebar.

Not doing so is annoying because we want some elements to be able to cover the navbar (announcementBar shadow, skipToContent)... and yet have the mobile navbar cover those.

I gave it a try in facebook/docusaurus#7940 to increase the announcementBar z-index so that its shadow can be seen on top of the navbar (gives a better sensation of depth and seems the right thing to do)

But this leads to issues on mobile:

• Announcement bar covers 'Skip to main content' button facebook/docusaurus#8173
• Banner covers menu on mobile facebook/docusaurus#8141

Due to the current inability of having 2 separate stacking contexts, I have to fix the 2 issues above by reverting my change, and removing the announcementBar shadow in favor of a bottom border.

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We should refactor Infima to allow rendering the navbar and its associated mobile sidebar in 2 separate DOM trees. Ie not having classes such as .navbar.navbar-sidebar--show .navbar-sidebar like we currently have, assuming the sidebar is a child of the navbar.

@yangshun if it were me I'd remove all things related to macro-layout orchestration like fixed/sticky positioning and margins from Infima and only declare these things on the Docusaurus side. Having this in Infima creates DX friction to test the change easily locally and in PRs. I'd rather have Infima focus only on the design system elements and not how elements are displayed inside the app. Any opinion?

Note that if we want to have a Tailwind theme, it's likely to have the same mobile sidebar behavior, animation and positions. I'd rather have this CSS in @docusaurus/theme-common as shared CSS modules so that we can avoid duplicating it in each design system / theme.