merge SolrConfigXml wiki into Solr Reference Guide

The default and example solrconfig.xml currently point to the https://wiki.apache.org/solr/SolrConfigXml page, might we change that perhaps to point into the Solr Reference Guide (and merge content from the wiki into the ref guide where appropriate)?

<!--
      For more details about configurations options that may appear in
-     this file, see http://wiki.apache.org/solr/SolrConfigXml.
+     this file, see https://lucene.apache.org/solr/guide/7_x/configuring-solrconfig-xml.html.
-->

Implementation detail: the 7_x in the above would need to be an actual 7_3 etc. version in each release.