Add missing multi-protocol file (#5519)

It was reported that the link for multi-protocol wasn't working:
#5454

After looking, the file is not present, looks like it got lost during
migration, this is the original file:
https://github.com/timotheeguerin/typespec/blob/663d057c08c496d0fb3a858c8a528e9900c05bde/packages/website/src/pages/multi-protocol.md

Adding the file again and verifying

website/src/pages/multi-protocol.md

@@ -0,0 +1,59 @@
+---
+layout: ../layouts/content.astro
+---
+
+# Multi protocol
+
+TypeSpec is a protocol agnostic language. It could be used with many different protocols independently or together.
+
+## Examples
+
+### Protobuf service and emit a json schema for the models.
+
+In this example we have a protobuf service and we want to emit a json schema for the models which we can use later to validate the data in our service implementation.
+
+```tsp tryit="{"emit": ["@typespec/protobuf", "@typespec/json-schema"]}"
+import "@typespec/protobuf";
+import "@typespec/http";
+import "@typespec/json-schema";
+
+using TypeSpec.Protobuf;
+using TypeSpec.Http;
+
+@JsonSchema.jsonSchema
+@Protobuf.package({
+  name: "kiosk",
+})
+@TypeSpec.service
+namespace KioskExample;
+
+// models.tsp
+model Kiosk {
+  @field(1) id?: int32;
+  @field(2) name: string;
+  @field(3) size: ScreenSize;
+  @field(4) location: LatLng;
+  @field(5) create_time?: int32;
+}
+model ScreenSize {
+  @field(1) width: int32;
+  @field(2) height: int32;
+}
+
+model LatLng {
+  @field(1) lat: float64;
+  @field(2) lng: float64;
+}
+
+model ListResponse {
+  @field(1) kiosks: Kiosk[];
+}
+
+@Protobuf.service
+interface Kiosks {
+  @post createKiosk(...Kiosk): Kiosk;
+  @list listKiosks(): ListResponse;
+  @get getKiosk(@path @field(1) id: int32): Kiosk;
+  @delete deleteKiosk(@path @field(1) id: int32): void;
+}
+```