Docs: Add plugin to list

This plugin provides trigram accelerated regular expression search for when
you need reasonably fast arbitrary matching.

Closes #7997

docs/reference/modules/plugins.asciidoc

@@ -321,6 +321,7 @@ You can disable that check using `plugins.check_lucene: false`.
 * https://github.com/sonian/elasticsearch-zookeeper[ZooKeeper Discovery Plugin] (by Sonian Inc.)
 * https://github.com/kzwang/elasticsearch-image[Elasticsearch Image Plugin] (by Kevin Wang)
 * https://github.com/wikimedia/search-highlighter[Elasticsearch Experimental Highlighter] (by Wikimedia Foundation/Nik Everett)
+* https://github.com/wikimedia/search-extra[Elasticsearch Trigram Accelerated Regular Expression Filter] (by Wikimedia Foundation/Nik Everett)
 * https://github.com/salyh/elasticsearch-security-plugin[Elasticsearch Security Plugin] (by Hendrik Saly)
 * https://github.com/codelibs/elasticsearch-taste[Elasticsearch Taste Plugin] (by CodeLibs Project)
 * http://siren.solutions/siren/downloads/[Elasticsearch SIREn Plugin]: Nested data search (by SIREn Solutions)