Add optional icon to CI templates

Problem to solve

For project templates, we show an icon today:

But .gitlab-ci.yml templates are just text:

It would be nice to have a little icon in there - maybe just in the same folder as the templates and names thesamething.png

Intended users

Further details

Proposal

Permissions and Security

Documentation

Testing

What does success look like, and how can we measure that?

Links / references

/cc @dimitrieh