[code samples] introduce Gradle composite build, fixed gradle.xml
[code samples] introduce Gradle composite build, fixed gradle.xml

artspb/intellij-sdk-docs,JetBrains/intellij-sdk-docs,artspb/intellij-sdk-docs,artspb/intellij-sdk-docs,artspb/intellij-sdk-docs,JetBrains/intellij-sdk-docs,JetBrains/intellij-sdk-docs