Fix a broken link in Markup Reference -> Code Blocks

[DiddiLeija]

Replace the old link (https://myst-parser.readthedocs.io/en/latest/using/intro.html#block-level-directives-with-myst-markdown) with the closest equivalent (https://myst-parser.readthedocs.io/en/latest/syntax/roles-and-directives.html).

Closes #593.