[Doc] Use Markdown syntax highlighting

A clean version of the https://github.com/symfony/symfony/pull/11999

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 
Given that the library used by GitHub to highlight code [supports Twig](https://github.com/github/linguist/blob/master/lib/linguist/languages.yml#L2362), this could be easily added to GitHub. I've just requested them to add support for `twig` highlighting and all of you can do the same at https://github.com/contact