fix(theme): mobile navbar & skipToContent should cover announcementBar

[adnanhashmi09]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

In mobile view the announcement banner covered the navbar. It has been fixed and the navbar is stacked on top so that the banner doesnt cover it anymore.

Test Plan

Test links

Deploy preview: https://deploy-preview-8163--docusaurus-2.netlify.app/

Related issues/PRs

Closes #8141

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 81	🟢 98	🟢 100	🟢 100	🟠 80	Report
/docs/installation	🟠 80	🟢 100	🟢 100	🟢 100	🟢 90	Report

[adnanhashmi09]

@slorber Please review this PR and let me know if you want any changes.

[slorber]

Thanks

Unfortunately it's not a good fix because it defeats the purpose of a change I did to make it possible to add a shadow to the announcement banner here: #7940

With this fix, the shadow is always covered by the navbar.

Will look for a better solution and edit a bit this PR

Also need to consider this: #8173

[slorber]

Thanks for giving it a try.

Unfortunately after studying the problem I come to the conclusion that some Infima refactor is required to achieve what I want (facebookincubator/infima#275) and the best solution is to temporarily revert my changes made here: #7940

Will do that in this PR so that you'll be credited for your attempt as well 👍

[adnanhashmi09]

Thank you for the review.
If there is anything I can do to help with this issue, do let me know.