First round of antora migration

docs/modules/ROOT/pages/intro.adoc

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+= Introduction
+
+Spring Cloud Function is a project with the following high-level goals:
+
+* Promote the implementation of business logic via functions.
+* Decouple the development lifecycle of business logic from any specific runtime target so that the same code can run as a web endpoint, a stream processor, or a task.
+* Support a uniform programming model across serverless providers, as well as the ability to run standalone (locally or in a PaaS).
+* Enable Spring Boot features (auto-configuration, dependency injection, metrics) on serverless providers.
+
+It abstracts away all of the transport details and
+infrastructure, allowing the developer to keep all the familiar tools
+and processes, and focus firmly on business logic.
+
+Here's a complete, executable, testable Spring Boot application
+(implementing a simple string manipulation):
+
+[source,java]
+----
+@SpringBootApplication
+public class Application {
+
+  @Bean
+  public Function<String, String> uppercase() {
+    return value -> value.toUpperCase();
+  }
+
+  public static void main(String[] args) {
+    SpringApplication.run(Application.class, args);
+  }
+}
+----
+
+It's just a Spring Boot application, so it can be built, run and
+tested, locally and in a CI build, the same way as any other Spring
+Boot application. The `Function` is from `java.util` and `Flux` is a
+https://www.reactive-streams.org/[Reactive Streams] `Publisher` from
+https://projectreactor.io/[Project Reactor]. The function can be
+accessed over HTTP or messaging.
+
+Spring Cloud Function has the following features:
+
+* _Choice of programming styles - reactive, imperative or hybrid._
+* _Function composition and adaptation (e.g., composing imperative functions with reactive)._
+* _Support for reactive function with multiple inputs and outputs allowing merging, joining and other complex streaming operation to be handled by functions._
+* _Transparent type conversion of inputs and outputs._
+* _Packaging functions for deployments, specific to the target platform (e.g., Project Riff, AWS Lambda and more)_
+* _Adapters to expose function to the outside world as HTTP endpoints etc._
+* _Deploying a JAR file containing such an application context with an isolated classloader, so that you can pack them together in a single JVM._
+* _Adapters for https://github.com/spring-cloud/spring-cloud-function/tree/master/spring-cloud-function-adapters/spring-cloud-function-adapter-aws[AWS Lambda], https://github.com/spring-cloud/spring-cloud-function/tree/master/spring-cloud-function-adapters/spring-cloud-function-adapter-azure[Azure], https://github.com/spring-cloud/spring-cloud-function/tree/master/spring-cloud-function-adapters/spring-cloud-function-adapter-gcp[Google Cloud Functions], and possibly other "serverless" service providers._