Fix Collapse reopening bug (#2290)

## Description

Fixed the bug reported on Discord:
https://discord.com/channels/625400653321076807/1453289347221557382

We previously used `Collapse` from `nextra/components` which didn't
remove the `width` when opening again.

This kinda seems like something new, because I opened and closed the
sidebar _a lot_ before and I never had this behavior, but I don't think
it's worth to investigate.

Author: hasparus