[7.x][DOCS] Adds intro text to the index file, changes snippet attributes

docs/connecting.asciidoc

@@ -1,10 +1,14 @@
 [[connecting]]
 == Connecting
 
+experimental[]
+
 The code snippet below shows how to initialize a low level REST client and the 
 Jackson object mapper to configure an ElasticsearchClient:
 
-```
+
+["source","java"]
+--------------------------------------------------
 // Create the low-level client
 RestClient restClient = RestClient.builder(new HttpHost("localhost", 9200)).build();
 
@@ -13,7 +17,7 @@ Transport transport = new RestClientTransport(restClient, new JacksonJsonpMapper
 
 // And create our API client
 ElasticsearchClient client = new ElasticsearchClient(transport);
-```
+--------------------------------------------------
 
 Authentication is managed by the 
 https://www.elastic.co/guide/en/elasticsearch/client/java-rest/current/java-rest-low.html[low-level Rest Client].

docs/index.asciidoc

@@ -1,5 +1,17 @@
 = Elasticsearch Java High-level client
 
+[partintro]
+--
+
+The {es} Java High-level REST Client is an experimental Java client for {es}. 
+It removes all dependencies to the {es} server code base.
+
+* <<introduction>>
+* <<installation>>
+* <<connecting>>
+
+--
+
 :branch: master
 include::{asciidoc-dir}/../../shared/attributes.asciidoc[]
 

docs/installation.asciidoc

@@ -1,6 +1,8 @@
 [[installation]]
 == Installation
 
+experimental[]
+
 This page guides you through the installation process of the client.
 
 Requirements:
@@ -25,7 +27,8 @@ https://snapshots.elastic.co/maven/[Elastic's Maven snapshot repository].
 [[gradle]]
 === Installation in a Gradle project by using Jackson
 
-```
+["source","groovy",subs="attributes"]
+--------------------------------------------------
 repositories {
     mavenCentral()
     maven {
@@ -38,7 +41,7 @@ dependencies {
     implementation 'co.elastic.clients:elasticsearch-java:7.15.0-SNAPSHOT'
     implementation 'com.fasterxml.jackson.core:jackson-databind:2.12.3'
 }
-```
+--------------------------------------------------
 
 [discrete]
 [[maven]]
@@ -47,7 +50,8 @@ dependencies {
 In the `pom.xml` of your project, add the following repository definition and 
 dependencies:
 
-```
+["source","xml",subs="attributes"]
+--------------------------------------------------
 <project>
 
   <repositories>
@@ -74,7 +78,7 @@ dependencies:
   </dependencies>
 
 </project>
-```
+--------------------------------------------------
 
 [discrete]
 [[compatibility]]