gitignore: ignore documentation build products

gitignore: ignore documentation build products

There are several different manuals by now, all of which produce
output. The "eclipse" directory might even have been present the last
time .gitignore was updated.

Ignore the files and directories that can be generated during "make
DOC=<directory>" as well as "make DOC=<directory> pdf". While at it,
make sure the patterns only match for the top-level documentation
directory.

Signed-off-by: Sascha Silbe <x-yo17@se-silbe.de>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>