doc: make it easier to find info about installation of old packages

doc/appendix-packages.sgml

@@ -456,14 +456,21 @@ repmgr96-4.1.1-0.0git320.g5113ab0.1.el7.x86_64.rpm</programlisting>
 
   <sect1 id="packages-old-versions" xreflabel="Installing old package versions">
     <title>Installing old package versions</title>
+
     <indexterm>
       <primary>old packages</primary>
     </indexterm>
+
     <indexterm>
       <primary>packages</primary>
       <secondary>old versions</secondary>
     </indexterm>
 
+    <indexterm>
+      <primary>installation</primary>
+      <secondary>old package versions</secondary>
+    </indexterm>
+
     <sect2 id="packages-old-versions-debian" xreflabel="old Debian package versions">
       <title>Debian/Ubuntu</title>
       <para>

doc/install-packages.sgml

@@ -1,5 +1,11 @@
 <sect1 id="installation-packages" xreflabel="Installing from packages">
  <title>Installing &repmgr; from packages</title>
+
+  <indexterm>
+   <primary>installation</primary>
+   <secondary>from packages</secondary>
+  </indexterm>
+
  <para>
   We recommend installing &repmgr; using the available packages for your
   system.
@@ -160,7 +166,17 @@ yum search repmgr</programlisting>
       <programlisting>
         [root@localhost ~]# yum install repmgr10-4.0.3-1.rhel7</programlisting>
     </para>
+
+    <para>
+      <emphasis>Installing old packages</emphasis>
+    </para>
+    <para>
+      See appendix <link linkend="packages-old-versions-rhel-centos">Installing old package versions</link>
+      for details on how to retrieve older package versions.
+    </para>
+
   </sect3>
+
  </sect2>
 
  <sect2 id="installation-packages-debian" xreflabel="Installing from packages on Debian or Ubuntu">
@@ -224,7 +240,6 @@ curl https://dl.2ndquadrant.com/default/release/get/deb | sudo bash</programlist
           </note>
         </listitem>
 
-
 	<listitem>
 	  <para>
             Install the &repmgr version appropriate for your PostgreSQL version (e.g. <literal>repmgr10</literal>):
@@ -244,6 +259,15 @@ sudo apt-get install postgresql-10-repmgr</programlisting>
 
     </para>
 
+    <para>
+      <emphasis>Installing old packages</emphasis>
+    </para>
+    <para>
+      See appendix <link linkend="packages-old-versions-debian">Installing old package versions</link>
+      for details on how to retrieve older package versions.
+    </para>
+
+
   </sect3>
  </sect2>