doc: add/translate/extend documentation of contexts, images, routes

commit 3b752509fd6ef78938064f3e9d5ef6840f06e66b
Author: Max F. Albrecht <1@178.is>
Date:   Mon Jun 30 18:07:19 2014 +0200

    doc: add table of image (thumbnail) sizes

commit 8abcdc4fd1bab4e4cb7468fb328df2ea59a4c6c8
Author: Max F. Albrecht <1@178.is>
Date:   Mon Jun 30 18:53:17 2014 +0200

    doc: translate the section about contexts & vocabularies to english

commit 3a3ef12d219cf769fd8940f8eb09352e00a0bc7d
Author: Max F. Albrecht <1@178.is>
Date:   Mon Jun 30 18:52:29 2014 +0200

    doc: cleanup 'routes' chapter