Convert reference documents written in Markdown to PDF

[ricola]

The three reference documents for the session "Tecnologías para el trabajo colaborativo" have their source files in Markdown (and PNG images) but are pointed to as direct downloads from the web interface, so unformatted and without images.

I prepared a branch convert-markdown-downloads-in-pdf that adds a Makefile to allow compiling the Markdown sources into PDF when needed. It also includes a build of these PDFs as of today.

Please tell me:

• If you think it's a good solution. I really think we should favor having the source of all these documents in a plain text format since the idea is to have them open for collaboration through Git.
• If the sources, Makefile, and PDF builds are in their appropriate location.

[lorenzode]

@alexandraTTC TTC dev team finished working on this, so I guess this is better in Alex's hands