doc: update wasi code sample

- Code sample updated by adding a hello-world (`demo.wat`) code example
- Step for compiling `.wat` to `.wasm` added (with reference to
  `wabt` tools)
- The sample code prints "hello world\n" in the console

Note: This update adds a very minimal change to the existing sample and
can be treated as an extension.

PR-URL: #33626
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Author: pragyandas

doc/api/wasi.md

@@ -22,15 +22,51 @@ const wasi = new WASI({
 const importObject = { wasi_snapshot_preview1: wasi.wasiImport };
 
 (async () => {
-  const wasm = await WebAssembly.compile(fs.readFileSync('./binary.wasm'));
+  const wasm = await WebAssembly.compile(fs.readFileSync('./demo.wasm'));
   const instance = await WebAssembly.instantiate(wasm, importObject);
 
   wasi.start(instance);
 })();
 ```
 
+To run the above example, create a new WebAssembly text format file i.e. `demo.wat`
+```wat
+(module
+    ;; Import the required fd_write WASI function which will write the given io vectors to stdout
+    ;; The function signature for fd_write is:
+    ;; (File Descriptor, *iovs, iovs_len, nwritten) -> Returns number of bytes written
+    (import "wasi_snapshot_preview1" "fd_write" (func $fd_write (param i32 i32 i32 i32) (result i32)))
+
+    (memory 1)
+    (export "memory" (memory 0))
+
+    ;; Write 'hello world\n' to memory at an offset of 8 bytes
+    ;; Note the trailing newline which is required for the text to appear
+    (data (i32.const 8) "hello world\n")
+
+    (func $main (export "_start")
+        ;; Creating a new io vector within linear memory
+        (i32.store (i32.const 0) (i32.const 8))  ;; iov.iov_base - This is a pointer to the start of the 'hello world\n' string
+        (i32.store (i32.const 4) (i32.const 12))  ;; iov.iov_len - The length of the 'hello world\n' string
+
+        (call $fd_write
+            (i32.const 1) ;; file_descriptor - 1 for stdout
+            (i32.const 0) ;; *iovs - The pointer to the iov array, which is stored at memory location 0
+            (i32.const 1) ;; iovs_len - We're printing 1 string stored in an iov - so one.
+            (i32.const 20) ;; nwritten - A place in memory to store the number of bytes written
+        )
+        drop ;; Discard the number of bytes written from the top of the stack
+    )
+)
+```
+Use [wabt](https://github.com/WebAssembly/wabt) to compile `.wat` to `.wasm`
+
+```sh
+$ wat2wasm demo.wat
+```
+
 The `--experimental-wasi-unstable-preview1` and `--experimental-wasm-bigint`
-CLI arguments are needed for the previous example to run.
+CLI arguments are needed for this example to run.
 
 ## Class: `WASI`
 <!-- YAML