Add the fact that Web IDE does syntax highlighting for all languages supported by Monaco

Update of existing documentation.

This may require adding some language about Web IDE and its features, since we don't do that very well here https://docs.gitlab.com/ee/user/project/web_ide/ -- but we should add that syntax highlighting is availabile for all these languages and can link out: https://github.com/Microsoft/monaco-languages/tree/master/src

Conversation that led to this https://gitlab.slack.com/archives/C0AR2KW4B/p1541016632153800

CC @jramsay