[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 if we can make Twig enter the list of template languages supported by Pygments, we'll be able to also use `twig`  highlighting in Symfony doc.

To support this, please vote on this issue: https://bitbucket.org/birkenfeld/pygments-main/issue/927/missing-lexer-for-twig-language  or help use review and finish this pull request: https://bitbucket.org/birkenfeld/pygments-main/pull-request/404/first-attempt-of-creating-a-twig-lexer/diff