Update dokka to 1.5.0

See spring-projectsgh-26870

build.gradle

@@ -313,6 +313,8 @@ configure([rootProject] + javaProjects) { project ->
 
 	pluginManager.withPlugin("kotlin") {
 		apply plugin: "org.jetbrains.dokka"
+		apply from: "${rootDir}/gradle/docs-dokka.gradle"
+
 		compileKotlin {
 			kotlinOptions {
 				languageVersion = "1.3"

gradle/docs-dokka.gradle

@@ -0,0 +1,30 @@
+tasks.findByName("dokkaHtmlPartial")?.configure {
+    outputDirectory.set(new File(buildDir, "docs/kdoc"))
+    dokkaSourceSets {
+        configureEach {
+            sourceRoots.setFrom(file("src/main/kotlin"))
+            classpath.from(sourceSets["main"].runtimeClasspath)
+            externalDocumentationLink {
+                url.set(new URL("https://docs.spring.io/spring-framework/docs/current/javadoc-api/"))
+            }
+            externalDocumentationLink {
+                url.set(new URL("https://projectreactor.io/docs/core/release/api/"))
+            }
+            externalDocumentationLink {
+                url.set(new URL("https://www.reactive-streams.org/reactive-streams-1.0.1-javadoc/"))
+            }
+            externalDocumentationLink {
+                url.set(new URL("https://kotlin.github.io/kotlinx.coroutines/"))
+            }
+            externalDocumentationLink {
+                url.set(new URL("http://hamcrest.org/JavaHamcrest/javadoc/2.1/"))
+            }
+            externalDocumentationLink {
+                url.set(new URL("https://javadoc.io/doc/javax.servlet/javax.servlet-api/latest/"))
+            }
+            externalDocumentationLink {
+                url.set(new URL("https://javadoc.io/static/io.rsocket/rsocket-core/1.1.1/"))
+            }
+        }
+    }
+}

settings.gradle

@@ -1,7 +1,7 @@
 pluginManagement {
 	repositories {
 		gradlePluginPortal()
-		maven { url 'https://repo.spring.io/plugins-release' }
+		maven { url "https://repo.spring.io/plugins-release" }
 	}
 }