Add a 'copy code to clipboard' button for code blocks

[byaruhaf]

The swift book has a number of code blocks to complement the explanations.
To improve the experience for the reader we should add a “copy code” button to make it simple for them to copy and paste the code.

[amartini51]

This is enhancement request for docc which is used by TSPL — transferring it to the Swift-DocC repository.

[natikgadzhi]

Related: apple/swift-docc#737

Both in code blocks — if someone wanted to make two improvements with one PR ;)

[mportiz08]

Thanks for the suggestion @byaruhaf. I believe we are already tracking this enhancement request in #274