chore(build): Add missing build files

[guillermocalvo]

Currently, CI is broken on branch 4.0.x.

* What went wrong:
A problem occurred evaluating project ':docs'.
> Could not get unknown property 'groovyVersion' for object of type 
org.gradle.api.internal.artifacts.dsl.dependencies.DefaultDependencyHandler.

I believe the cause of the problem is that some Gradle files were accidentally deleted due to a merge conflict.