RFC: default UI polishing

### Have you read the Contributing Guidelines on issues?

- [X] I have read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/main/CONTRIBUTING.md#reporting-new-issues).

### Description

We have several UI polishing requests that I think are very justified. Although they can be easily implemented through swizzling, I do think Docusaurus should offer a better default UI.

1. The language icon doesn't look very good. The [React docs](https://reactjs.org/) uses the `mdiTranslate` material icon, which looks sturdier.
2. The color mode toggle should be a button rather than a toggle. This can be argued both ways (the toggle looks more explicit), but seems like a button is by far the most prevalent choice: [Material UI](https://mui.com/), [VuePress](https://v2.vuepress.vuejs.org/), [Ionic](https://ionicframework.com/docs) (which is still Docusaurus!)
3. The sidebar collapse button should be on the side instead of at the bottom. For example, [Dyte](https://docs.dyte.io/docs/home/introduction). The reason is because there's a significant deviation in the position of the toggle button when the sidebar is expanded vs. collapsed. In addition, the button at the very bottom of the page is admittedly not very visible.

This doesn't mean that everything needs to be implemented, just want to hear about more people's opinions on what UI improvements we should make.

### Has this been requested on Canny?

https://docusaurus.canny.io/feature-requests/p/make-the-language-changer-icon-configurable

### Self-service

- [X] I'd be willing to contribute this feature to Docusaurus myself.