Rollup merge of rust-lang#69049 - pthariensflame:improvement/imgbot, r=GuillaumeGomez

Optimize image sizes

This was done by [ImgBot](https://imgbot.net/) on my own fork, which I then immediately merged and turned into this manual pull request. Below is reproduced [ImgBot's own message](pthariensflame#3 (comment)) on the content of this PR.

r? @steveklabnik since this is a documentation PR, I guess.

---

> ## Beep boop. Your images are optimized!
>
> Your image file size has been reduced by **21%** 🎉
>
> <details>
> <summary>
> Details
> </summary>
>
> | File | Before | After | Percent reduction |
> |:--|:--|:--|:--|
> | /src/etc/installer/gfx/rust-logo.png | 5.71kb | 3.82kb | 33.11% |
> | /src/librustdoc/html/static/down-arrow.svg | 0.63kb | 0.50kb | 20.44% |
> | /src/librustdoc/html/static/wheel.svg | 3.86kb | 3.68kb | 4.66% |
> | /src/librustdoc/html/static/brush.svg | 0.47kb | 0.44kb | 4.61% |
> | | | | |
> | **Total :** | **10.65kb** | **8.44kb** | **20.82%** |
> </details>
>
> ---
>
> [📝docs](https://imgbot.net/docs) | [:octocat: repo](https://github.com/dabutvin/ImgBot) | [🙋issues](https://github.com/dabutvin/ImgBot/issues) | [🏅swag](https://goo.gl/forms/1GX7wlhGEX8nkhGO2) | [🏪marketplace](https://github.com/marketplace/imgbot)

Author: Dylan-DPC