[DOCS] Adds missing anchor in Installation Upgrade Guide (#222)

Author: lcawl

docs/en/install-upgrade/index.asciidoc

@@ -7,28 +7,7 @@
 include::{asciidoc-dir}/../../shared/attributes.asciidoc[]
 include::{es-repo-dir}/Versions.asciidoc[]
 
-== Overview
-
-The products in the https://www.elastic.co/products[{stack}]
-are designed to be used together and releases are synchronized
-to simplify the installation and upgrade process. The full stack
-consists of:
-
-* {beats-ref}/index.html[Beats {branch}]
-* {apm-server-ref}/index.html[APM Server {branch}]
-* {ref}/index.html[Elasticsearch {branch}]
-* {hadoop-ref}/index.html[Elasticsearch Hadoop {branch}]
-* {kibana-ref}/index.html[Kibana {branch}]
-* {logstash-ref}/index.html[Logstash {branch}]
-
-This guide provides information about installing and upgrading
-when you are using more than one {stack} product. It specifies
-the recommended order of installation and the steps you need to take
-to prepare for a stack upgrade.
-
-For detailed information about breaking changes in {version} and install
-and upgrade instructions for specific components, see the individual
-product reference guides.
+include::overview.asciidoc[]
 
 include::installing-stack.asciidoc[]
 

docs/en/install-upgrade/overview.asciidoc

@@ -0,0 +1,23 @@
+[[overview]]
+== Overview
+
+The products in the https://www.elastic.co/products[{stack}]
+are designed to be used together and releases are synchronized
+to simplify the installation and upgrade process. The full stack
+consists of:
+
+* {beats-ref}/index.html[Beats {branch}]
+* {apm-server-ref}/index.html[APM Server {branch}]
+* {ref}/index.html[Elasticsearch {branch}]
+* {hadoop-ref}/index.html[Elasticsearch Hadoop {branch}]
+* {kibana-ref}/index.html[Kibana {branch}]
+* {logstash-ref}/index.html[Logstash {branch}]
+
+This guide provides information about installing and upgrading
+when you are using more than one {stack} product. It specifies
+the recommended order of installation and the steps you need to take
+to prepare for a stack upgrade.
+
+For detailed information about breaking changes in {version} and install
+and upgrade instructions for specific components, see the individual
+product reference guides.