Make txt2tags/doc a submodule for this repo

[aureliojargas]

We are cloning "by hand" the txt2tags/doc repo on the build.sh script.

I think using this other repo as a submodule here will make it clearer that it is a dependency for the site. Also, Netlify will already clone the submodule, so we won't need that extra step in our build script.

[fgallaire]

Hello, I was thinking exactly the same thing last night. Step by step it seems we are finding a good architecture. Remain the @farvardin concerns about the visibility of the doc folder as a second level submodule directory. For me this is the right choice with visibility thanks to an autonomous pinned repository.