doc: pdf: Use single column for the index

The index at the end of the PDF document can contain pretty long strings
that don't fit in the default 2-column layout.
This commit makes the index use a single-column.

Note: this relies on LaTeX package idxlayout which should already be
included in the packages recommended for installation in our
instructions.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>