Add a list of SQL keywords to the ref guide (#7449)

* Add a list of SQL keywords to the ref guide

* Fix error from review

* Update from review comment

gpdb-doc/dita/GPRefGuide.ditamap

@@ -208,6 +208,8 @@
 	<chapter href="ref_guide/SQL2008_support.xml"/>
 	<chapter href="ref_guide/env_var_ref.xml" navtitle="Greenplum Environment Variables"
 		id="env_var_ref"/>
+	<chapter href="ref_guide/sql-keywords.xml" navtitle="Reserved Identifiers and SQL Key Words"
+		id="keyword_ref"/>
 	<chapter href="ref_guide/system_catalogs/catalog_ref.xml">
 		<topicref href="ref_guide/system_catalogs/catalog_ref-tables.xml"/>
 		<topicref href="ref_guide/system_catalogs/catalog_ref-views.xml"/>

gpdb-doc/dita/ref_guide/preface.xml

@@ -47,6 +47,9 @@
     <li>
      <xref href="env_var_ref.xml#topic1"/>
     </li>
+    <li>
+      <xref href="sql_keywords.xml#topic1"/>
+    </li>
     <li id="ac168706">
      <xref href="system_catalogs/catalog_ref.xml#topic1"/>
     </li>

gpdb-doc/dita/ref_guide/ref_guide.ditamap

@@ -165,6 +165,7 @@
 		</topicref>
 		<topicref href="SQL2008_support.xml"/>
 		<topicref href="env_var_ref.xml" navtitle="Greenplum Environment Variables" id="env_var_ref"/>
+		<topicref href="sql-keywords.xml" navtitle="Reserved Identifiers and SQL Keywords" id="sql_keywords"/>
 		<topicref href="system_catalogs/catalog_ref.xml#topic1">
 			<topicref href="system_catalogs/catalog_ref-tables.xml#topic1"/>
 			<topicref href="system_catalogs/catalog_ref-views.xml#topic1"/>