Documentation generation

[Laeeth]

D now allows html and markdown format documentation so I think to maintain its non standard (is there a standard for documentation macros?) is now out of date. Maybe you can say its different from doxygen etc but it seems a strange reason to pick or reject a language.

https://dlang.org/spec/ddoc.html#highlighting

[timotheecour]

PRs welcome; I wrote this before D allowed for markdown in doc