add docs about script.js processor

src/content/docs/processors/script/js.mdx

@@ -0,0 +1,23 @@
+---
+title: JavaScript
+sidebar:
+    order: 1
+---
+
+The `script.js` processor takes in any payload type and executes a JavaScript function that is able to process the payload. The end value of `payload` will be returned as the output of the processor.
+
+Ending with a `payload` value of `undefined` is treated the same as any other processor returning `nil`.
+
+This run your JavaScript program in a [sandboxed environment](https://gitlab.com/cznic/quickjs). It currently support the [ES2023](https://tc39.es/ecma262/2023/) specification.
+
+- **type**: `script.js`
+- **params**:
+    - **program**: The JavaScript program to run. The payload is available as a global variable called `payload`.
+### Example
+Replace all occurrences of `|` with `,` in the payload
+```
+- type: script.js
+  params:
+    program: |
+        payload = payload.replaceAll('|',',');
+```