Bumped doc versions to 6.0.0-alpha1

docs/java-api/index.asciidoc

@@ -1,7 +1,7 @@
 [[java-api]]
 = Java API
 :ref: http://www.elastic.co/guide/en/elasticsearch/reference/master
-:version: 5.0.0-alpha5
+:version: 6.0.0-alpha1
 
 [preface]
 == Preface

docs/java-rest/index.asciidoc

@@ -1,7 +1,7 @@
 [[java-rest]]
 = Java REST Client
 
-:version: 5.0.0-alpha5
+:version: 6.0.0-alpha1
 
 include::overview.asciidoc[]
 

docs/plugins/index.asciidoc

@@ -2,7 +2,7 @@
 
 :ref:            https://www.elastic.co/guide/en/elasticsearch/reference/master
 :guide:          https://www.elastic.co/guide
-:version:        5.0.0-alpha5
+:version:        6.0.0-alpha1
 :lucene_version: 6.2.0
 
 [[intro]]

docs/reference/index.asciidoc

@@ -1,8 +1,8 @@
 [[elasticsearch-reference]]
 = Elasticsearch Reference
 
-:version:        5.0.0-alpha5
-:major-version:  5.x
+:version:        6.0.0-alpha1
+:major-version:  6.x
 :lucene_version: 6.2.0
 :branch:         master
 :jdk:            1.8.0_73