Serialization doc updated (#58575)

This PR updates the documentation for Serialization.jl to recommend
using the .jls extension for serialized Julia data, following common
community conventions.

Fixes: #58544

Author: arnavk23

stdlib/Serialization/docs/src/index.md

@@ -11,3 +11,15 @@ Serialization.serialize
 Serialization.deserialize
 Serialization.writeheader
 ```
+
+### Recommended File Extension
+
+While the Serialization module does not mandate a specific file extension, the Julia community commonly uses the `.jls` extension for serialized Julia files.
+
+Example:
+
+```julia
+open("model.jls", "w") do io
+    serialize(io, my_model)
+end
+```