Add copy to clipboard button for code snippets

[eljhkrr]

While going through the v4 setup guide, I had to manually copy text blocks containing shell commands:

It would be very useful if users could have a copy to clipboard button to grab the whole set of commands at a go like in the example below:

There are a number of workarounds both in the hugo ecosystem and using vanilla JS that can be adopted to add this functionality. A few examples:
https://digitaldrummerj.me/hugo-add-copy-code-snippet-button
https://www.dannyguo.com/blog/how-to-add-copy-to-clipboard-buttons-to-code-blocks-in-hugo/

[eljhkrr]

Copy code snippet is available in docsy v0.6.0+
google/docsy#1447
https://www.docsy.dev/docs/adding-content/lookandfeel/#code-highlighting-with-chroma

[esthermmoturi]

Done!