Split system and administrative docs

Hi @matias78 and @AlphaGit, I have moved some documents, anyway, there is still work to do as you can see in Trello: [Manual de sistema](https://trello.com/c/DuRWcUQe/105-manual-de-sistema) and [Manual de administrador](https://trello.com/c/OM0a4Iyp/106-manual-de-administrador).

I am not sure why there are `index.markdown` files in anywhere, but I updated topic rendering to ignore them.
Not important, just a curiosity: turns out that `descrito` is used everywhere, but `descripto` is used in Argentina and Uruguay. (At least, that's what I understand from [RAE](http://lema.rae.es/drae/?val=descripto))