docs(contributing): update the CONTRIBUTING.md catala code examples

CONTRIBUTING.md

@@ -33,12 +33,12 @@ copy-paste the text into your source file.
 First you will have to format the copy-pasted text using Catala headings
 and articles markers:
 
-```
-@@Heading@@
+```markdown
+## Heading
 
-@@Sub-heading (the more +, the less important)@@++
+### Sub-heading (the more +, the less important)
 
-@Legislative atom@
+#### [Legislative atom]
 ```
 
 Please look at the code of other examples to see how to format things properly.
@@ -54,30 +54,32 @@ to see if you've made any syntax errors. Once the text formatting is done, you
 can start to annotate each legislative atom (article, provision, etc.) with
 some Catala code. To open up a code section in Catala, simply use
 
-```
-/*
+~~~markdown
+```catala
 # In code sections, comments start with #
 scope Foo:
   <your code goes here>
-*/
 ```
+~~~
 
 While all the code sections are equivalent in terms of execution, you can
 mark some as "metadata" so that they are printed differently on lawyer-facing
 documents. Here's how it works:
 
-```
-@@Begin metadata@@ # @@Début métadonnées@@ en français
-/*
+~~~markdown
+> Begin metadata # > Début métadonnées en français
+
+```catala
 declaration structure FooBar:
   data foo content boolean
   data bar content money
 
 <your structure/enumeration/scope declarations goes here>
-*/
-@@End metadata@@ # @@Fin métadonnées@@ en français
 ```
 
+> End metadata # > Fin métadonnées en français
+~~~
+
 Again, make sure to regularly check that your example is parsing correctly. The error message from the compiler should help you debug the syntax if need be. You can also
 live-test the programs you wrote by feeding them to the interpreter
 (see the [README of the `examples` directory](examples/README.md)); this will