docs - initial doc content for Greenplum R client beta (#9772)

* docs - initial doc content for Greenplum R client beta

* use correct download fname, qualify version as Beta

* add more functions to the table, move html help info

gpdb-doc/dita/analytics/analytics.ditamap

@@ -26,4 +26,5 @@
       <topicref href="pl_python.xml" linking="none"/>
       <topicref href="pl_r.xml" linking="none"/>
     </topicref>
+    <topicref href="greenplum_r_client.xml" linking="none"/>
 </map>