doc: tweak .gitignore to allow doc theme dev

doc processing wants themes to be in the doc/themes folder
This patch makes it easier to develop and test the website
theme with local content generation usint the web theme.

Change-Id: Icc615f2ecbe3b7cecb7974405a44304ffdf68966
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>