[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        | 
GitHub has responded to my question: `twig` doesn't work because they use the Pygments library and it's not supported yet.

There is an issue in the Pygments repository for adding Twig. Please vote for it to get more attention: https://bitbucket.org/birkenfeld/pygments-main/issue/927/missing-lexer-for-twig-language