dev-manual, ref-manual: Created new section on initramfs

Fixes [YOCTO #7096]

We did not document how to create an initramfs image to be included
with a kernel build.  Various variables sort of inferred the
knowledge.  I created a new section in the "Common Tasks" section
of the dev-manual that describes how to create an initramfs image.

Also, I updated the kernel.bbclass reference section to point back
to the new "how-to" section.

Finally, I also created a bunch of cross-reference links from various
related variables back to the new "how-to" section.

(From yocto-docs rev: a14e550494641c46ac2518632cbf251e07d459fd)

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