[wasm][docs] Add docs for using AOT profiler (#47170)

* add instructions for using profiler

* [wasm][docs] add link to wasm/build/README.md

Author: tqiu8

src/mono/netcore/sample/wasm/browser-profile/README.md

@@ -0,0 +1,69 @@
+## How to run a sample app with AOT profiling enabled
+
+### Setting up a project with profiling
+
+1. Define a `write_at` method. By default it is:
+
+```
+[MethodImpl(MethodImplOptions.NoInlining)]
+ public static void StopProfile(){}
+```
+
+2. Initialize the profiler in the main javascript (e.g. runtime.js)
+
+```
+var Module = {
+  onRuntimeInitialized: function () {
+    ...
+    
+    if (config.enable_profiler)
+    {
+      config.aot_profiler_options = {
+        write_at: "<Namespace.Class::StopProfile>",
+        send_to: "System.Runtime.InteropServices.JavaScript.Runtime::DumpAotProfileData"
+    }
+  }
+```
+
+3. Call the `write_at` method at the end of the app, either in C# or in JS. To call the `write_at` method in JS, make use of bindings:
+
+`BINDING.call_static_method("<[ProjectName] Namespace.Class::StopProfile">, []);`
+
+When the `write_at` method is called, the `send_to` method `DumpAotProfileData` stores the profile data into `Module.aot_profile_data`
+
+4. Download `Module.aot_profile_data` in JS, using something similar to:
+
+```
+function saveProfile() {
+  var a = document.createElement('a');
+  var blob = new Blob([Module.aot_profile_data]);
+  a.href = URL.createObjectURL(blob);
+  a.download = "data.aotprofile";
+  // Append anchor to body.
+  document.body.appendChild(a);
+  a.click();
+
+  // Remove anchor from body
+  document.body.removeChild(a);
+}
+```
+
+### Build and Run a project with profiling
+1. To enable profiling during a build, we need to make use of WasmApp.InTree.targets/props by importing into the project file:
+
+`<Import Project="$(MonoProjectRoot)\wasm\build\WasmApp.InTree.targets" />` <br/>
+`<Import Project="$(MonoProjectRoot)wasm\build\WasmApp.InTree.props" />` 
+
+For more information on how to utilize WasmApp.InTree.targets/props consult the wasm build directory [README.md](../../../../wasm/build/README.md)
+
+2. To get the profile data, run: 
+
+`make get-aot-profile`
+
+Which will build and run the current project with AOT disabled and the AOT profiler enabled.
+
+3. Go to localhost:8000 and the profile will automatically download. 
+
+4. To use the profile data in the project, run:
+
+`make use-aot-profile PROFILE_PATH=<path to profile file>`