Move the SDK demo to sdk-target-based-on-call-view-model

The `sdk/` directory is about to become the MatrixRTC SDK package, so the
demo that lives there — one bundle of Element Call's CallViewModel for a plain
page or widget — moves aside, unchanged. It keeps building under a script and
vite config of its own, `pnpm build:sdk-target-based-on-call-view-model`, into
its own `dist` rather than the repository root's, and no longer copies the
developer's config.json out of `public`. Its README says which script to run
and where the output lands.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

vite-sdk.config.ts

@@ -1,28 +0,0 @@
-/*
-Copyright 2025 Element Creations Ltd.
-
-SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
-Please see LICENSE in the repository root for full details.
-*/
-
-import { defineConfig, mergeConfig } from "vite";
-import nodePolyfills from "vite-plugin-node-stdlib-browser";
-
-const base = "./";
-
-// Config for embedded deployments (possibly hosted under a non-root path)
-export default defineConfig(() => ({
-  worker: { format: "es" as const },
-  base, // Use relative URLs to allow the app to be hosted under any path
-  build: {
-    sourcemap: true,
-    manifest: true,
-    lib: {
-      formats: ["es" as const],
-      entry: "./sdk/main.ts",
-      name: "MatrixrtcSdk",
-      fileName: "matrixrtc-sdk",
-    },
-  },
-  plugins: [nodePolyfills()],
-}));