Add set index decorator syntax

Author: Kevan Stannard

misc_docs/syntax/decorator_set_index.mdx

@@ -0,0 +1,72 @@
+---
+id: "set-index-decorator"
+keywords: ["set", "decorator", "array", "object"]
+name: "@set_index"
+summary: "This is the `@set_index` decorator."
+category: "decorators"
+---
+
+The `@set_index` decorator is used to set a dynamic property or index.
+
+For example with an Array:
+
+<CodeTab labels={["ReScript", "JS Output"]}>
+
+```res
+type t
+
+@new external create: int => t = "Array"
+@set_index external set: (t, int, string) => unit = ""
+@get_index external get: (t, int) => string = ""
+
+let a = create(3)
+a->set(0, "zero")
+a->set(1, "one")
+a->set(2, "two")
+
+let value = a->get(1)
+```
+
+```js
+var a = new Array(3);
+
+a[0] = "zero";
+a[1] = "one";
+a[2] = "two";
+
+var value = a[1];
+```
+
+</CodeTab>
+
+And an example with an object:
+
+<CodeTab labels={["ReScript", "JS Output"]}>
+
+```res
+type t
+
+@new external create: unit => t = "Object"
+@set_index external set: (t, string, int) => unit = ""
+@get_index external get: (t, string) => int = ""
+
+let o = create()
+o->set("x", 1)
+o->set("y", 3)
+o->set("z", 5)
+
+let value = o->get("y")
+```
+
+```js
+var o = new Object();
+
+o["x"] = 1;
+o["y"] = 3;
+o["z"] = 5;
+
+var value = o["y"];
+```
+
+</CodeTab>
+