Allow users to change docs version on mobile

[srdtrk]

Summary of Bug

Users can't switch between versioned docs on mobile.

Expected Behaviour

There should be a frontend element to allow users to switch between versions without manually modifying the URL.

Description of Bug

Docusaurus v3 allows users to switch versions from the homepage on mobile, but we have removed the standard landing page, so now, there is no way for users to switch versions.

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For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged/assigned

[womensrights]

mention this in #6502