documentation: Created new YP Overview Manual

Fixes [YOCTO #12370]

Added a new chapter to the newly created Yocto Project Overview
Manual.  This chapter originated from the YP Reference Manual and
was the old chapter 3, which talked a lot about various concepts.
This information is better suited for the new overview manual.

The change involved moving the entire chapter and renaming it.
This move of the content affected many, many external references
and links into the old area.  Consequently, I had to recast all
these links from the different manual.

Changes also included fixing the mega-manual.xml file so that it
would include the new overview manual as part of it.

Many figures had to be relocated as part of the move as well.  This
meant deleting them from the ref-manual/figures folder and putting
them into the overview-manual/figures folder.

(From yocto-docs rev: 87b81358f2bbd02b4a0d966d86c4d7b006d4d78f)

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

documentation/bsp-guide/bsp.xml

@@ -55,7 +55,7 @@
                 To help understand the BSP layer concept, consider the BSPs that the
                 Yocto Project supports and provides with each release.
                 You can see the layers in the
-                <ulink url='&YOCTO_DOCS_REF_URL;#yocto-project-repositories'>Yocto Project Source Repositories</ulink>
+                <ulink url='&YOCTO_DOCS_OVERVIEW_URL;#yocto-project-repositories'>Yocto Project Source Repositories</ulink>
                 through a web interface at
                 <ulink url='&YOCTO_GIT_URL;/cgit/cgit.cgi'></ulink>.
                 If you go to that interface, you will find near the bottom of the list
@@ -1486,7 +1486,7 @@
 
                 <para>
                     Designed to have a  command interface somewhat like
-                    <ulink url='&YOCTO_DOCS_REF_URL;#git'>Git</ulink>, each
+                    <ulink url='&YOCTO_DOCS_OVERVIEW_URL;#git'>Git</ulink>, each
                     tool is structured as a set of sub-commands under a
                     top-level command.
                     The top-level command (<filename>yocto-bsp</filename>