Fix code boxes to use the Copy button

[riusricardo]

A new functionality was implemented in the docs but it requires to have instructions on its own box or use an operator to handle multiples commands.

substrate-node-new substrate-node-template <author-name>
substrate-ui-new substrate

Change to:

substrate-node-new substrate-node-template <author-name>

substrate-ui-new substrate

Or:

substrate-node-new substrate-node-template <author-name> && substrate-ui-new substrate