docs(README): next.js logo with dark mode (#40223)
This PR follows #40181, adding the dark mode support for the brand new
Next.js logo (BTW, the new logo is awesome!). The dark version of the
logo is also served from `assets.vercel.com` (I just change the original
logo URL, replace the `light` with `dark`, and surprisingly find out
that doesn't result in 404).
cc @Nutlope @steven-tey
Ref: [Specify theme context for images in Markdown (Beta) - GitHub
Blog](https://github.blog/changelog/2022-05-19-specify-theme-context-for-images-in-markdown-beta/)
Currently, the Next.js README in dark mode:
<img width="917" alt="image"
src="https://user-images.githubusercontent.com/40715044/188449078-864f6b71-7741-44f4-aee6-02a281b0a263.png">
After the PR:
<img width="914" alt="image"
src="https://user-images.githubusercontent.com/40715044/188449265-a3556565-d866-4e0e-9b97-be62fba6c6fe.png">
Preview link:
https://github.com/SukkaW/next.js/tree/readme-logo-darkmode
