Add titles to codeblocks and switch from diff codeblock to language codeblock

Signed-off-by: Paul Schultz <pschultz@pobox.com>

docs/auth/google/gcp-iap-auth.md

@@ -45,7 +45,7 @@ callbacks in actual code as well as described below.
 Add a `providerFactories` entry to the router in
 `packages/backend/src/plugins/auth.ts`.
 
-```ts
+```ts title="packages/backend/src/plugins/auth.ts"
 import { providers } from '@backstage/plugin-auth-backend';
 import { stringifyEntityRef } from '@backstage/catalog-model';
 
@@ -99,12 +99,18 @@ sign-in mechanism to poll that endpoint through the IAP, on the user's behalf.
 It is recommended to use the `ProxiedSignInPage` for this provider, which is
 installed in `packages/app/src/App.tsx` like this:
 
-```diff
-+import { ProxiedSignInPage } from '@backstage/core-components';
+```tsx title="packages/app/src/App.tsx"
+/* highlight-add-next-line */
+import { ProxiedSignInPage } from '@backstage/core-components';
 
- const app = createApp({
-   components: {
-+    SignInPage: props => <ProxiedSignInPage {...props} provider="gcp-iap" />,
+const app = createApp({
+ /* highlight-add-start */
+  components: {
+    SignInPage: props => <ProxiedSignInPage {...props} provider="gcp-iap" />,
+  },
+  /* highlight-add-end */
+  // ..
+})
 ```
 
 See the [Sign-In with Proxy Providers](../index.md#sign-in-with-proxy-providers) section for more information.