[Feature] Add dark mode on yarnpkg website

[trivikr]

• [ ] I'd be willing to implement this feature (contributing guide)
• [ ] This feature is important to have in this repository; a contrib plugin wouldn't do

Describe the user story

The website https://yarnpkg.com/ does not have dark mode

Describe the solution you'd like

• Add dark mode for the website.
• Automatically load a dark or light theme based on system preferences.
• Allow the user to manually override their preference using a toggle in the title/navigation bar, and maintain preferred theme between page reloads.

Some gatsby plugins:

• gatsby-plugin-dark-mode
• gatsby-plugin-use-dark-mode

Describe the drawbacks of your solution

I don't see a drawback. Folks who want to see the website in light mode can always select it.

Describe alternatives you've considered

• Reduce brightness of the screen while reading yarnpkg website.
• Use a browser extension to enforce dark mode on the website.
• Revisit the website in the mornings when system has light mode enabled.

[trivikr]

The legacy Gatsby website was removed in https://github.com/yarnpkg/berry/pull/5742

The new website is built using Docusaurus 2, and code can be viewed at https://github.com/yarnpkg/berry/tree/master/packages/docusaurus

Docusaurus allows configuring light/dark themes using colorMode configuration. Existing website colorMode configuration: https://github.com/yarnpkg/berry/blob/ffa193c59c6de53be20773a9c1958d3dc0d48221/packages/docusaurus/docusaurus.config.js#L95-L99