message: docs: add sign-in resolvers in getting started auth
diff: @@ -79,6 +79,7 @@ Now let's look at the example, with the rest of the commentary being made with i
  the code comments:
  ```ts
  +// File: packages/backend/src/plugins/auth.ts
  import {
  createRouter,
  providers,
  @@ -139,11 +140,21 @@ property of each of the auth provider integrations. For example, the Google prov
  a built in resolver that works just like the one we defined above:
  ```ts
  -providers.google.create({
  +// File: packages/backend/src/plugins/auth.ts
  +export default async function createPlugin(
  + // ...
  + return await createRouter({
  + // ...
  + providerFactories: {
  + // ...
  + google: providers.google.create({
  signIn: {
  resolver: providers.google.resolvers.emailLocalPartMatchingUserEntityName(),
  },
  });
  + }
  + })
  +)
  ```
  There are also other options, like the this one that looks up a user
  @@ -164,10 +175,16 @@ that happens during sign-in you can replace `ctx.signInWithCatalogUser` with a s
  of lower-level calls:
  ```ts
  +// File: packages/backend/src/plugins/auth.ts
  import { getDefaultOwnershipRefs } from '@backstage/plugin-auth-backend';
  -// This example only shows the resolver function itself.
  -async ({ profile: { email } }, ctx) => {
  +export default async function createPlugin(
  + // ...
  + return await createRouter({
  + // ...
  + providerFactories: {
  + // ...
  + google: async ({ profile: { email } }, ctx) => {
  if (!email) {
  throw new Error('User profile contained no email');
  }
  @@ -198,6 +215,9 @@ async ({ profile: { email } }, ctx) => {
  },
  });
  };
  + }
  + })
  +)
  ```
  ## Sign-In without Users in the Catalog
  @@ -221,10 +241,16 @@ check like in the example below, or you might for example look up the GitHub org
  that the user belongs to using the user access token in the provided result object.
  ```ts
  +// File: packages/backend/src/plugins/auth.ts
  import { DEFAULT_NAMESPACE, stringifyEntityRef, } from '@backstage/catalog-model';
  -// This example only shows the resolver function itself.
  -async ({ profile }, ctx) => {
  +export default async function createPlugin(
  + // ...
  + return await createRouter({
  + // ...
  + providerFactories: {
  + // ...
  + google: async ({ profile }, ctx) => {
  if (!profile.email) {
  throw new Error(
  'Login failed, user profile does not contain an email',
  @@ -253,6 +279,9 @@ async ({ profile }, ctx) => {
  },
  });
  },
  + }
  + })
  +)
  ```
  ## AuthHandler
  