[DOCS] Add 8.15.1 release notes (#2258) (#2259)

(cherry picked from commit b6df69e)

Author: jbaiera

docs/src/reference/asciidoc/appendix/release-notes/8.15.1.adoc

@@ -0,0 +1,5 @@
+[[eshadoop-8.15.1]]
+== Elasticsearch for Apache Hadoop version 8.15.1
+
+ES-Hadoop 8.15.1 is a version compatibility release, tested specifically against
+Elasticsearch 8.15.1.

docs/src/reference/asciidoc/appendix/release.adoc

@@ -9,6 +9,7 @@ This section summarizes the changes in each release.
 [[release-notes-8]]
 ===== 8.x
 
+* <<eshadoop-8.15.1>>
 * <<eshadoop-8.15.0>>
 * <<eshadoop-8.14.3>>
 * <<eshadoop-8.14.2>>
@@ -112,6 +113,7 @@ Elasticsearch 5.3.1.
 
 ////////////////////////
 
+include::release-notes/8.15.1.adoc[]
 include::release-notes/8.15.0.adoc[]
 include::release-notes/8.14.3.adoc[]
 include::release-notes/8.14.2.adoc[]