documentation: Updated the section on creating a general script

documentation: Updated the section on creating a general script

Split this section into two sections: one for creating the layer
using bitbake-layers create-layer and one for adding the layer
to bblayers.conf using bitbake-layers add-layer.

Needed to update some references in the yocto-project-qs and
kernel-dev manuals.

(From yocto-docs rev: 741a29b0cbeaaeaa0ac9155036ace2623938aee3)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>