Doc: Reorganize overview and installation chapters

* [DOCS] Reorganizes Overview and Installation chapters.
* [DOCS] Changes leveloffset.
* [DOCS] Changes installed versions.

Closes elastic#185

.doc/index.asciidoc

@@ -1,70 +1,9 @@
-= go-elasticsearch
+= Elasticsearch Go Client
 
-== Overview
+:doctype:           book
 
-An official Go client for Elasticsearch.
+include::{asciidoc-dir}/../../shared/attributes.asciidoc[]
 
-Full documentation is hosted at https://github.com/elastic/go-elasticsearch[GitHub]
-and https://godoc.org/github.com/elastic/go-elasticsearch[GoDoc]
--- this page provides only an overview.
+include::overview.asciidoc[]
 
-=== Elasticsearch Version Compatibility
-
-The client major versions correspond to the Elasticsearch major versions:
-to connect to Elasticsearch `6.x`, use a `6.x` version of the client,
-to connect to Elasticsearch `7.x`, use a `7.x` version of the client, and so on.
-
-The `master` branch of the client is compatible with the `master` branch of Elasticsearch.
-
-=== Installation
-
-Add the package to your `go.mod` file:
-
-[source,text]
-------------------------------------
-require github.com/elastic/go-elasticsearch/v7 7.x
-------------------------------------
-
-=== Usage
-
-[source,go]
-------------------------------------
-package main
-
-import (
-  "log"
-
-  "github.com/elastic/go-elasticsearch/v7"
-)
-
-func main() {
-  es, _ := elasticsearch.NewDefaultClient()
-  log.Println(es.Info())
-}
-------------------------------------
-
-[NOTE]
-Please have a look at the collection of comprehensive examples in the repository
-at https://github.com/elastic/go-elasticsearch/tree/master/_examples.
-
-== Resources
-
-* https://github.com/elastic/go-elasticsearch[Source Code]
-* https://godoc.org/github.com/elastic/go-elasticsearch[API Documentation]
-* https://github.com/elastic/go-elasticsearch/tree/master/_examples[Examples and Recipes]
-
-== License
-
-Copyright 2019 Elasticsearch
-
-Licensed under the Apache License, Version 2.0 (the "License");
-you may not use this file except in compliance with the License.
-You may obtain a copy of the License at
-
-    http://www.apache.org/licenses/LICENSE-2.0
-
-Unless required by applicable law or agreed to in writing, software
-distributed under the License is distributed on an "AS IS" BASIS,
-WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-See the License for the specific language governing permissions and
-limitations under the License.
+include::installation.asciidoc[]

.doc/installation.asciidoc

@@ -0,0 +1,50 @@
+[[installation]]
+== Installation
+
+To install the 7.x version of the client, add the package to your `go.mod` file:
+
+[source,text]
+------------------------------------
+require github.com/elastic/go-elasticsearch/v7 7.x
+------------------------------------
+
+Or, clone the repository:
+
+[source,text]
+------------------------------------
+git clone --branch 7.x https://github.com/elastic/go-elasticsearch.git $GOPATH/src/github
+------------------------------------
+
+To install another version, modify the path or the branch name accordingly. The 
+client major versions correspond to the {es} major versions.
+
+You can find a complete example of installation below:
+
+[source,text]
+------------------------------------
+mkdir my-elasticsearch-app && cd my-elasticsearch-app
+
+cat > go.mod <<-END
+  module my-elasticsearch-app
+
+  require github.com/elastic/go-elasticsearch/v8 master
+END
+
+cat > main.go <<-END
+  package main
+
+  import (
+    "log"
+
+    "github.com/elastic/go-elasticsearch/v8"
+  )
+
+  func main() {
+    es, _ := elasticsearch.NewDefaultClient()
+    log.Println(elasticsearch.Version)
+    log.Println(es.Info())
+  }
+END
+
+go run main.go
+------------------------------------

.doc/overview.asciidoc

@@ -0,0 +1,70 @@
+[[overview]]
+== Overview
+
+This is the official Go client for {es}.
+
+Full documentation is hosted at 
+https://github.com/elastic/go-elasticsearch[GitHub]
+and https://godoc.org/github.com/elastic/go-elasticsearch[GoDoc]. This 
+documentation provides only an overview of features.
+
+
+[discrete]
+=== {es} Version Compatibility
+
+The client major versions correspond to the {es} major versions:
+to connect to {es} `6.x`, use a `6.x` version of the client,
+to connect to {es} `7.x`, use a `7.x` version of the client, and so on.
+
+The `master` branch of the client is compatible with the `master` branch of 
+{es}.
+
+
+[discrete]
+=== Usage
+
+[source,go]
+------------------------------------
+package main
+
+import (
+  "log"
+
+  "github.com/elastic/go-elasticsearch/v7"
+)
+
+func main() {
+  es, _ := elasticsearch.NewDefaultClient()
+  log.Println(es.Info())
+}
+------------------------------------
+
+[NOTE]
+Please have a look at the collection of comprehensive examples in the repository
+at https://github.com/elastic/go-elasticsearch/tree/master/_examples.
+
+
+[discrete]
+=== Resources
+
+* https://github.com/elastic/go-elasticsearch[Source Code]
+* https://godoc.org/github.com/elastic/go-elasticsearch[API Documentation]
+* https://github.com/elastic/go-elasticsearch/tree/master/_examples[Examples and Recipes]
+
+
+[discrete]
+=== License
+
+Copyright 2019 {es}.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.