add a docstring to Registry module (#4432)

Author: KristofferC

src/Registry/Registry.jl

@@ -1,3 +1,41 @@
+"""
+    Pkg.Registry
+
+A module for managing Julia package registries.
+
+Registries are repositories that contain metadata about available packages, including
+their versions, dependencies, and locations. The most common registry is the General
+registry, which hosts publicly available Julia packages.
+
+# Main Functions
+
+- [`Pkg.Registry.add`](@ref): Add new package registries
+- [`Pkg.Registry.rm`](@ref): Remove installed registries
+- [`Pkg.Registry.update`](@ref): Update installed registries
+- [`Pkg.Registry.status`](@ref): Display information about available registries
+
+# Examples
+
+```julia
+# Add the default registries (typically the General registry)
+Pkg.Registry.add()
+
+# Add a specific registry by name, UUID, or URL
+Pkg.Registry.add("General")
+Pkg.Registry.add(url = "https://github.com/JuliaRegistries/General.git")
+
+# Update all registries
+Pkg.Registry.update()
+
+# Check registry status
+Pkg.Registry.status()
+
+# Remove a registry
+Pkg.Registry.rm("General")
+```
+
+See also: [`RegistrySpec`](@ref)
+"""
 module Registry
 
 import ..Pkg