Add signature in node
If the "signature" doesn't contain any special programmatical behavior and its basically textual (whether styled with CSS and/or images or not)

Than I have the ultimate simple solution for you:

Create a block with the desired content, and place it under your "Content" block on your main content region by going to Structure > Block layout.

The signature-block will now appear under every page/node (you could filter where it would appear by means of content type or NID -> Node ID number).