README: Hide old Gradle plugin instructions, clear up setup flow (#2470)

Author: SebastianAigner

README.md

@@ -23,9 +23,8 @@ Kotlin serialization consists of a compiler plugin, that generates visitor code
 * [Introduction and references](#introduction-and-references)
 * [Setup](#setup)
   * [Gradle](#gradle)
-    * [Using the `plugins` block](#using-the-plugins-block)
-    * [Using `apply plugin` (the old way)](#using-apply-plugin-the-old-way)
-    * [Dependency on the JSON library](#dependency-on-the-json-library)
+    * [Setting up the serialization plugin](#1-setting-up-the-serialization-plugin)
+    * [Dependency on the JSON library](#2-dependency-on-the-json-library)
   * [Android](#android)
   * [Multiplatform (Common, JS, Native)](#multiplatform-common-js-native)
   * [Maven](#maven)
@@ -83,9 +82,13 @@ Make sure you have the corresponding Kotlin plugin installed in the IDE, no addi
 
 ### Gradle
 
-#### Using the `plugins` block
+To set up kotlinx.serialization, you have to do two things:
+1) Add the **[serialization plugin](#1-setting-up-the-serialization-plugin)**.
+2) Add the **[serialization library dependency](#2-dependency-on-the-json-library)**.
 
-You can set up the serialization plugin with the Kotlin plugin using 
+#### 1) Setting up the serialization plugin
+
+You can set up the serialization plugin with the Kotlin plugin using the
 [Gradle plugins DSL](https://docs.gradle.org/current/userguide/plugins.html#sec:plugins_block):
 
 Kotlin DSL:
@@ -106,9 +109,10 @@ plugins {
 }
 ```
 
-> Kotlin versions before 1.4.0 are not supported by the stable release of Kotlin serialization
+> Kotlin versions before 1.4.0 are not supported by the stable release of Kotlin serialization.
 
-#### Using `apply plugin` (the old way)
+<details>
+  <summary>Using <code>apply plugin</code> (the old way)</summary>
 
 First, you have to add the serialization plugin to your classpath as the other [compiler plugins](https://kotlinlang.org/docs/reference/compiler-plugins.html):
 
@@ -145,10 +149,11 @@ Then you can `apply plugin` (example in Groovy):
 apply plugin: 'kotlin' // or 'kotlin-multiplatform' for multiplatform projects
 apply plugin: 'kotlinx-serialization'
 ```
+</details>
 
-#### Dependency on the JSON library
+#### 2) Dependency on the JSON library
 
-After setting up the plugin one way or another, you have to add a dependency on the serialization library.
+After setting up the plugin, you have to add a dependency on the serialization library.
 Note that while the plugin has version the same as the compiler one, runtime library has different coordinates, repository and versioning.
 
 Kotlin DSL: