Support Dokka HTML customization (Kotlin#2008)

Author: nikpachoo

build.gradle

@@ -188,3 +188,7 @@ subprojects {
 apply from: rootProject.file('gradle/compiler-version.gradle')
 apply from: rootProject.file("gradle/dokka.gradle")
 apply from: rootProject.file("gradle/benchmark-parsing.gradle")
+
+tasks.named("dokkaHtmlMultiModule") {
+    pluginsMapConfiguration.set(["org.jetbrains.dokka.base.DokkaBase": """{ "templatesDir": "${projectDir.toString().replace('\\', '/')}/dokka-templates" }"""])
+}

dokka-templates/README.md

@@ -0,0 +1,12 @@
+# Dokka's template customization
+To provide unified navigation for all parts of [kotlinlang.org](https://kotlinlang.org/),
+the Kotlin Website Team uses this directory to place custom templates in this folder
+during the website build time on TeamCity.
+
+It is not practical to place these templates in the kotlinx.serialization repository because they change from time to time
+and aren't related to the library's release cycle.
+
+The folder is defined as a source for custom templates by the templatesDir property through Dokka's plugin configuration.
+
+[Here](https://kotlin.github.io/dokka/1.7.20-SNAPSHOT/user_guide/output-formats/html/#custom-html-pages), you can
+find more about the customization of Dokka's HTML output.

gradle/dokka.gradle

@@ -20,6 +20,8 @@ subprojects {
 
     tasks.named('dokkaHtmlPartial') {
         outputDirectory = file("build/dokka")
+        pluginsMapConfiguration.set(["org.jetbrains.dokka.base.DokkaBase": """{ "templatesDir": "${rootProject.projectDir.toString().replace('\\', '/')}/dokka-templates" }"""])
+
         dokkaSourceSets {
             configureEach {
                 includes.from(rootProject.file('dokka/moduledoc.md').path)