Csongor Bokay

This PR adds support for [Solidity](https://en.wikipedia.org/wiki/Solidity) (`.sol`) syntax highlighting. - [x] I tested the changes in this pull request myself - [x] I updated the README.md, with pictures if necessary

Currently CPV only supports vaccination and test result certificates, we need to add support for recovery certificates too.

This can be used to determine if someone is fit-for-travel/-entry based on their vaccination, test or recovery status. These business rules can vary by country.