Add an email signature example to the tools-and-tips page

In the tools-and-tips section I want to add an email signature example to make it easier for the reader to setup a GitLab email signature.

Checklist when modifying an existing page:

What is the goal of changing the page? To make it easier for the reader to setup a GitLab email signature.
Would we revert the changes if we decided that it was not a success? Yes