Add serverless documentation

Add a documentation describing how to connect to a ScyllaDB Cloud
serverless database. The documentation is based on the existing "Quick
start" manual:

https://java-driver.docs.scylladb.com/scylla-3.11.2.x/manual/index.html

Author: avelanarius

manual/README.md

@@ -315,6 +315,7 @@ simply navigate to each sub-directory.
    async/*
    auth/*
    compression/*
+   serverless/*
    control_connection/*
    custom_codecs/*
    custom_payloads/*

manual/serverless/README.md

@@ -0,0 +1,46 @@
+## Connecting to ScyllaDB Cloud Serverless
+
+With ScyllaDB Cloud, you can deploy [serverless databases](https://cloud.docs.scylladb.com/stable/serverless/index.html).
+The Java driver allows you to connect to a serverless database by utilizing the connection bundle you can download via the **Connect>Java** tab in the Cloud application.
+The connection bundle is a YAML file with connection and credential information for your cluster.
+
+Connecting to a ScyllaDB Cloud serverless database is very similar to a standard connection to a ScyllaDB database.
+
+Here’s a short program that connects to a ScyllaDB Cloud serverless database and executes a query:
+
+```java
+Cluster cluster = null;
+try {
+    File bundleFile = new File("/file/downloaded/from/cloud/connect-bundle.yaml");
+        
+    cluster = Cluster.builder()                                                  // (1)
+            .withScyllaCloudConnectionConfig(bundleFile)                         // (2)
+            .build();
+    Session session = cluster.connect();                                         // (3)
+
+    ResultSet rs = session.execute("select release_version from system.local");  // (4)
+    Row row = rs.one();
+    System.out.println(row.getString("release_version"));                        // (5)
+} finally {
+    if (cluster != null) cluster.close();                                        // (6)
+}
+```
+
+1. The [Cluster] object is the main entry point of the driver. It holds the known state of the actual ScyllaDB cluster
+   (notably the [Metadata](metadata/)). This class is thread-safe, you should create a single instance (per target
+   ScyllaDB cluster), and share it throughout your application;
+2. [withScyllaCloudConnectionConfig] is a method that configures the cluster endpoints and credentials
+   to your ScyllaDB Cloud serverless cluster based on the YAML connection bundle you downloaded from ScyllaDB Cloud;
+3. The [Session] is what you use to execute queries. Likewise, it is thread-safe and should be reused;
+4. We use `execute` to send a query to Cassandra. This returns a [ResultSet], which is essentially a collection of [Row]
+   objects. On the next line, we extract the first row (which is the only one in this case);
+5. We extract the value of the first (and only) column from the row;
+6. Finally, we close the cluster after we're done with it. This will also close any session that was created from this
+   cluster. This step is important because it frees underlying resources (TCP connections, thread pools...). In a real
+   application, you would typically do this at shutdown (for example, when undeploying your webapp).
+
+[withScyllaCloudConnectionConfig]: https://java-driver.docs.scylladb.com/scylla-3.11.2.x/api/com/datastax/driver/core/Cluster.Builder.html#withScyllaCloudConnectionConfig-java.io.File-
+[Cluster]: https://docs.datastax.com/en/drivers/java/3.11/com/datastax/driver/core/Cluster.html
+[Session]: https://docs.datastax.com/en/drivers/java/3.11/com/datastax/driver/core/Session.html
+[ResultSet]: https://docs.datastax.com/en/drivers/java/3.11/com/datastax/driver/core/ResultSet.html
+[Row]: https://docs.datastax.com/en/drivers/java/3.11/com/datastax/driver/core/Row.html