[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        | 
@stof you are right. I was confused by this sentence of the official GitHub documentation:

> We use Linguist to perform language detection and syntax highlighting.

Source: https://help.github.com/articles/github-flavored-markdown#syntax-highlighting