Add doc/ structure

Add structure to `doc/` as described in issue https://github.com/telefonicaid/fiware-connectors/issues/83.

This kind of work is needed before working on documentation issues such as https://github.com/telefonicaid/fiware-connectors/issues/105.

Such a structure is:

    doc/
     |__design
         |__OrionCKANSink.md
         |__OrionMySQLSink.md
         |__OrionHDFSSink.md
         |__architecture.md
         |__ha.md
     |__devel
         |__add_new_sink.md
     |__installation
         |__chef_install.md
         |__rpm_install.md
         |__src_install.md
     |__operation
         |__ngsi2cosmos_migration.md
     |__quick_start_guide.md

All the documents has a "TBD" text.