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/sdk-manual/sdk-appendix-obtain.xml

@@ -327,7 +327,7 @@
                 <para>Following is an example command that extracts the root
                 filesystem from a previously built root filesystem image that
                 was downloaded from the
-                <ulink url='&YOCTO_DOCS_REF_URL;#index-downloads'>Index of Releases</ulink>.
+                <ulink url='&YOCTO_DOCS_OVERVIEW_URL;#index-downloads'>Index of Releases</ulink>.
                 This command extracts the root filesystem into the
                 <filename>core2-64-sato</filename> directory:
                 <literallayout class='monospaced'>

documentation/sdk-manual/sdk-extensible.xml

@@ -247,8 +247,8 @@
         <para>
             The <filename>devtool</filename> command line is organized
             similarly to
-            <ulink url='&YOCTO_DOCS_REF_URL;#git'>Git</ulink> in that it has a
-            number of sub-commands for each function.
+            <ulink url='&YOCTO_DOCS_OVERVIEW_URL;#git'>Git</ulink> in that it
+            has a number of sub-commands for each function.
             You can run <filename>devtool --help</filename> to see all the
             commands.
             <note>