(docs) Cookbook "Validate an Email Address" with re (#2407)

* Cookbook getenv\n\nRecreate PR #2383

* Update data/cookbook/read-environment-variable/00-stdlib.ml

* Apply suggestions from code review

---------

Co-authored-by: Cuihtlauac ALVARADO <[email protected]>
Co-authored-by: sabine <[email protected]>

data/cookbook/read-environment-variable/00-stdlib.ml

@@ -0,0 +1,26 @@
+---
+packages: []
+---
+
+(*
+  Both `Sys.getenv` and `Sys.getenv_opt` are functions that take the name of an environment and read its value.
+
+  `Sys.getenv` returns the value directly, but raises a `Not_found` exception if the variable doesn't exist.
+*)
+let path = Sys.getenv "PATH"
+let () =
+  try
+    Printf.printf "The path is %s\n" path
+  with Not_found ->
+    print_string "Api key is not set.\n"
+
+(*
+  In contrast, `Sys.getenv_opt` returns a value of type `string option`: `Some value` if the variable exists and `None` if it doesn't.
+*)
+let () =
+  match Sys.getenv_opt "API_KEY" with
+  | Some p ->
+      Printf.printf "Api key is %s\n" p
+  | None ->
+      print_string "Api key is not set.\n"
+