[Docs]: customize copy code button

[lukaspie]

If we have something like

user@box:~$ dataconverter generate-template --nxdl NXmynxdl

in the docs, the "copy code" option will also copy "user@box". We should customize the codebox so only dataconverter generate-template --nxdl NXmynxdl gets copied.

There seems to be some solution along the lines of changing the extras.css file:

.codehilite .no-copy {
    user-select: none;       /* Prevents text selection */
    pointer-events: none;    /* Prevents interaction */
    color: #6a737d;          /* Optional: style the prefix to look like a prompt */
}
´``
and then using
```console
<span class="no-copy">user@box:~$</span> dataconverter generate-template --nxdl NXmynxdl

But this does not yet work out of the box.