[BUG] `tex` and `texminted` formats doesn't produce valid tex files

[aviatesk]

(separated from #308 )

our formatter (, well, "formatter" is honestly really bad naming, rather we should call "renderer") doesn't do any task to convert our internal format (WeaveDoc) to tex file, while there is such a conversion done for md2tex format. (if interested, see https://github.com/JunoLab/Weave.jl/blob/b8b9b095b7d2196a04fb8d2b868743b5143d55ba/src/format.jl and check there is only format_hoge functions defined for md2tex and md2html formats) I think we need to define/overload functions to render tex for tex and texminted formats there as well, and then they will produce valid tex files.

/cc @JonasIsensee