[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        | 
@javiereguiluz linguist is not about highlighting, but about guessing the highlighting for files. As you can see in the code you linked, linguist does the same: it uses the jinja highlighter for .twig files.