[documentation][a11y] Add "copy code to clipboard" button for copying code snippets

[hydrosquall]

Enhancement Description

We could add a button could add a button so that copying JSON snippets from the docs is easier, especially for folks navigating docs without easy ability to click-and-drag (e.g. coding with voice, with trackpad instead of mouse)

Inspired by Salma's post, which also contains a reference JS implementation: https://whitep4nth3r.com/blog/how-to-build-a-copy-code-snippet-button/

If it worked out in vega-lite, we could propose a similar change in Vega too.

Potential scope

This could be applied to just examples where the JSON is stands on its own

Or to blocks where the code snippet is partial

[domoritz]

Makes sense but doesn't feel high priority right now to be honest. I think making copying too easy could be even problematic for partial snippets where the user needs to make changes.