doc: fix backslash paths in windows and python default location

Backslashes need to be doubled up, otherwise they are interpreted like
a escape sequence and basically, dissapear. As part of this, suggest
the location of the default python installation *before* using it, for
simplicity.

Change-Id: I26ed80a7dd8c08277614dc49a3986848892ff844
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>