docs: test for `access_token` in auth0 guide

Currently, we only have instructions to test the Auth0 login if we use Auth0.js (which generates an `id_token`): https://docs.hasura.io/1.0/graphql/manual/guides/integrations/auth0-jwt.html#test-auth0-login-and-generate-sample-jwts-for-testing.

We need to come up with a way to test the same also if we use `auth0-spa-js` (which generates an `access_token`).

Read the existing “Test Auth0 login and generate sample JWTs for testing” section in the linked Auth0 JWT guide, which documents Auth0.js and id_token testing. Determine how auth0-spa-js produces an access_token, then update the guide with equivalent testing instructions and verify that the access-token flow is documented clearly.

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