docs build: allow to specify custom dokka repository

- centralize repository management in settings.gradle.kts
- allow to specify dokka repository with a gradle property
- automatically add mavenLocal for -local dokka versions
- specify kotlin version for building plugins in one place

libraries/tools/kotlin-stdlib-docs/build.gradle.kts

@@ -50,11 +50,6 @@ val prepare by tasks.registering {
     dependsOn(":kotlin_big:extractLibs")
 }
 
-repositories {
-    mavenCentral()
-    maven(url = "https://maven.pkg.jetbrains.space/kotlin/p/dokka/dev")
-}
-
 dependencies {
     dokkaPlugin(project(":plugins:dokka-samples-transformer-plugin"))
     dokkaPlugin(project(":plugins:dokka-stdlib-configuration-plugin"))