Add copy button to examples

[besteves4]

Minor functionality update to HTML documents to have a copy button to easily copy-paste RDF examples.

[coolharsh55]

Hi. Please review if this is what you want: https://dev.dpvcg.org/2.1-dev/dpv/#example-nesting-processes and https://dev.dpvcg.org/2.1-dev/examples/#E0008 (if it doesn't show up, do a hard refresh or disable cache in browser network interface)

[besteves4]

It's perfect! :)

[bact]

Tested on Chrome, Safari, Firefox, Vivaldi, all good.

[coolharsh55]

Thanks @bact - this issue will be automatically closed when the commit is merged in master.