Add ability to add extra documentation pages

This CL adds a new homepage for the kmp docs that gets included
in the main page.
In a followup CL, i'll add the ability to have package docs.

Bug: 249096220
Test: prod :o

Change-Id: If23ec47d882835a5284477c140520fe08a35d935

Author: yigit

buildSrc/private/src/main/kotlin/androidx/build/dokka/kmpDocs/DokkaCombinedDocsTask.kt

@@ -79,6 +79,10 @@ internal abstract class DokkaCombinedDocsTask @Inject constructor(
     @get:OutputDirectory
     abstract val outputDir: DirectoryProperty
 
+    @get:InputFiles
+    @get:PathSensitive(PathSensitivity.RELATIVE)
+    abstract val additionalDocumentation: Property<FileCollection>
+
     @TaskAction
     fun buildCombinedDocs() {
         // create output dir before calling dokka to make sure relative links work.
@@ -114,7 +118,7 @@ internal abstract class DokkaCombinedDocsTask @Inject constructor(
             metadata to exportDir
         }
         val input = MergeDocsInputs(
-            moduleName = "AndroidX KMP EAP Docs",
+            moduleName = "Jetpack Multiplatform Preview Reference Documentation",
             outputDir = outputDir,
             pluginsClasspath = pluginsClasspath.get(),
             modules = partialModules.map { (module, docsDir) ->
@@ -127,7 +131,8 @@ internal abstract class DokkaCombinedDocsTask @Inject constructor(
             },
             pluginsConfiguration = listOf(
                 PluginsConfiguration.ANDROIDX_COPYRIGHT
-            )
+            ),
+            includes = additionalDocumentation.get().files
         )
         docsJsonOutput.get().asFile.let {
             it.parentFile.mkdirs()
@@ -177,6 +182,9 @@ internal abstract class DokkaCombinedDocsTask @Inject constructor(
                 it.dokkaCliClasspath.set(
                     DokkaUtils.createCliJarConfiguration(project)
                 )
+                it.additionalDocumentation.set(
+                    project.files("homepage.md")
+                )
             }
         }
     }

buildSrc/private/src/main/kotlin/androidx/build/dokka/kmpDocs/DokkaInputModels.kt

@@ -42,7 +42,8 @@ internal object DokkaInputModels {
         val moduleName: String,
         val pluginsClasspath: FileCollection,
         val modules: List<Module>,
-        val pluginsConfiguration: List<PluginsConfiguration>
+        val pluginsConfiguration: List<PluginsConfiguration>,
+        val includes: Set<File>
     )
 
     class Module(

docs-kmp/homepage.md

@@ -0,0 +1,6 @@
+# Jetpack Multiplatform Preview Reference Documentation
+
+This repository contains the reference documentation for the experimental Jetpack [Kotlin Multiplatform](https://kotlinlang.org/docs/multiplatform.html) preview libraries.
+
+* [Announcement Blog Post](https://android-developers.googleblog.com/2022/10/announcing-experimental-preview-of-jetpack-multiplatform-libraries.html)
+* [Sample Application](https://github.com/android/kotlin-multiplatform-samples/tree/main/DiceRoller)