Rename index file from indef.fr.txt to index.txt to dit Sphinx needs and adjusted conf file in consequence. Moved images from fr/ to book/ so that images would be in central location for both en and fr books.

msgid ""
msgstr ""
"Content-Type: text/plain; charset=UTF-8\n"
"Content-Transfer-Encoding: 8bit\n"
"Generated-By: pygettext.py 1.5\n"
"Plural-Forms: nplurals=2; plural=(n > 1);\n"