Added docstring for Artifacts.jl (JuliaLang#52913)

This is a part of issue JuliaLang#52725.

---------

(cherry picked from commit 15e2af2)
Co-Authored-By: Steven G. Johnson <stevenj@mit.edu>
Co-Authored-By: Max Horn <max@quendi.de>

stdlib/Artifacts/src/Artifacts.jl

@@ -1,5 +1,11 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
+"""
+Artifacts.jl is a Julia module that is used for managing and accessing
+artifacts in Julia packages. Artifacts are containers for
+platform-specific binaries, datasets, text, or any other kind of data
+that would be convenient to place within an immutable, life-cycled datastore.
+"""
 module Artifacts
 
 import Base: get, SHA1