[DOCS] Adds Release Notes for 8.9

Author: picandocodigo

docs/guide/release_notes/89.asciidoc

@@ -0,0 +1,10 @@
+[[release_notes_89]]
+=== 8.9 Release notes
+
+[discrete]
+[[release_notes_890]]
+=== 8.9.0 Release notes
+
+- Refactored User-Agent code, it is now set up on initializing the client instead of every request.
+- Tested versions of Ruby for 8.9.0: Ruby (MRI) 3.0, 3.1 and 3.2, JRuby 9.3 and JRuby 9.4.
+- Updated for compatibility with Elastic Enterprise Search 8.9's API.

docs/guide/release_notes/index.asciidoc

@@ -4,6 +4,7 @@
 [discrete]
 === 8.x
 
+* <<release_notes_89, 8.9.0 Release Notes>>
 * <<release_notes_88, 8.8.0 Release Notes>>
 * <<release_notes_87, 8.7.0 Release Notes>>
 * <<release_notes_86, 8.6.0 Release Notes>>
@@ -26,6 +27,7 @@
 * <<release_notes_711, 7.11.0 Release Notes>>
 * <<release_notes_710, 7.10.0.beta.1 Release Notes>>
 
+include::89.asciidoc[]
 include::88.asciidoc[]
 include::87.asciidoc[]
 include::86.asciidoc[]