Add a hello_world reference

doc/docs/reference/dune

@@ -0,0 +1,5 @@
+(mdx
+ (deps
+  (package provider)
+  (glob_files *.txt))
+ (preludes prelude.txt))

doc/docs/reference/hello_world.md

@@ -0,0 +1,35 @@
+# Hello World
+
+Below is a minimalist program that uses the Provider library (cheat sheet).
+
+```ocaml
+module type S = sig
+  type t
+
+  val show : t -> string
+end
+
+type show = [ `Show ]
+
+type (_, _, _) Provider.Trait.t +=
+  Show : ('t, (module S with type t = 't), [> show ]) Provider.Trait.t
+
+let print (Provider.T { t; interface }) =
+  let module M = (val Provider.Interface.lookup interface ~trait:Show) in
+  print_endline (M.show t)
+
+let string_provider t =
+  let interface =
+    Provider.Interface.make
+      [ Provider.Trait.implement Show
+          ~impl:(module struct type t = string let show = Fun.id end)
+      ]
+  in
+  Provider.T { t; interface }
+```
+
+```ocaml
+# print (string_provider "Hello World")
+Hello World
+- : unit = ()
+```

doc/docs/reference/prelude.txt

@@ -0,0 +1,2 @@
+#require "unix" ;;
+#require "provider" ;;

doc/sidebars.ts

@@ -39,6 +39,7 @@ const sidebars: SidebarsConfig = {
       items: [
         { type: 'doc', id: 'reference/odoc', label: 'OCaml Packages' },
         { type: 'doc', id: 'reference/glossary', label: 'Glossary' },
+        { type: 'doc', id: 'reference/hello_world', label: 'Hello World' },
       ],
     },
   ],