[BSv5] Navbar: add inner container + cleanup

[chalin]

• Contributes to Upgrade to Bootstrap 5.2 #470
• Adds an inner container to the navbar, as required by the migration guide.
• Runs Prettier over assets/scss/_nav.scss. It is best to view changes by ignoring whitespace differences
• Contributes to Cleanup: don't hard-code styles into layouts #783 by either moving custom styling into the corresponding classes, or dropping it in favor of BS defaults.

Preview: https://deploy-preview-1422--docsydocs.netlify.app

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Screenshots

Navbar scrolling still works on mobile:

[deining]

LGTM

[geriom]

Thank you! LGTM

[chalin]

Thanks for the reviews all!