docs - add resgroup links to best practices memory mgmt page
<title>Memory and Resource Management with Resource Queues</title>
<shortdesc>Avoid memory errors and manage Greenplum Database resources.</shortdesc>
<body>
+ <note>Resource groups are a newer resource management scheme that enforces memory, CPU, and concurrent transaction limits in Greenplum Database. The <xref href="../admin_guide/wlmgmt.xml" format="dita" type="topic" scope="peer" >Managing Resources</xref> topic provides a comparison of the resource queue and the resource group management schemes. Refer to <xref href="../admin_guide/workload_mgmt_resgroups.xml" format="dita" type="topic" scope="peer">Using Resource Groups</xref> for configuration and usage information for this resource management scheme.</note>
<p>Memory management has a significant impact on performance in a Greenplum Database cluster.
The default settings are suitable for most environments. Do not change the default settings
until you understand the memory characteristics and usage on your system. </p>
