docs: Fix the broken logo in README
Commit ebed63a8 changed the icon, but missed the reference in the documentation. It looks like gitlab doesn't support SVG natively, so add an appropriately sized png and point the documentation to it.
This is the fix from !105 (closed), but with the following changes:
- rebase to master
- don't install the png icon that is only used in the README
- follow commit message style
The original attribution is kept in place.