dev-manual, ref-manual: Added section on index of releases procedure

Fixes [YOCTO #11630]

I have added a new procedure on using the Index of Releases area
to get snapshots of various tarballs.  I recommend that is not
the way you usually want to get files as using Git is recommended.
The section exists for completeness.

In the ref-manual, I cross-reference the procedures from the
associated conceptual areas.

(From yocto-docs rev: 812f344966966591d883fbebb4ae1f72c11dd483)

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

documentation/ref-manual/ref-development-environment.xml

@@ -687,6 +687,10 @@
 
                 <para>
                 <imagedata fileref="figures/source-repos.png" align="center" width="6in" depth="4in" />
+                For steps on how to view and access these upstream Git
+                repositories, see the
+                "<ulink url='&YOCTO_DOCS_DEV_URL;#accessing-source-repositories'>Accessing Source Repositories</ulink>"
+                Section in the Yocto Project Development Manual.
                 </para></listitem>
             <listitem><para><anchor id='index-downloads' />
                 <emphasis>
@@ -703,6 +707,9 @@
 
                 <para>
                 <imagedata fileref="figures/index-downloads.png" align="center" width="6in" depth="3.5in" />
+                For steps on how to view and access these files, see the
+                "<ulink url='&YOCTO_DOCS_DEV_URL;#accessing-index-of-releases'>Accessing Index of Releases</ulink>"
+                section in the Yocto Project Development Manual.
                 </para></listitem>
             <listitem><para>
                 <emphasis>"Downloads" page for the