Add a sample of how to use immarkdown

[ptxmac]

I've been trying to use immarkdown, but something seems missing.
the Markdown method needs a MarkdownConfig object, but there's no way to create one correctly and the program just fails with a nil access

If it's supposed to work as is, please add a small sample demonstrating how

[gucio321]

Unfortunately as I said in release notes immarkdown is not fully functional as it misses callbacks impl for now.
Turns out that imgui_markdown doesn't provide NewMarkdownConfig function.
Maybe cmd/codegen/gengo_typedefs.go should also generate CreateEmptyXXX for all types?

func CreateEmptyXXX() XXX {
   return XXX{
      CData: new(C.XXX),
   }
}

@AllenDang what do you think about it?

Anyway, this will not work even if we add such a functions because then it'll panic for some reason. I suppose it needs to have FormatCallback set.

[AllenDang]

@gucio321 I don't have experience about immarkdown, so I cannot give an useful comment here...

[gucio321]

well, I think that overall, such a function would be useful.