Add Auth JS Example

[mraible]

[robertjd]

@mraible can you tell us more about what this example is for/what the discovery mechanism is? The README in the PR doesn't tell me what this example is for

[mraible]

@robertjd I created this as part of the Vue + Sign In Widget guide I'm working on. https://github.com/okta/okta-developer-docs/issues/1440

When I dug in to add the guide, I realized the first page in the other guides is an Auth JS + guide. I felt that any code we put in documentation should have a sample to back it up. That way, it's easier to update (b/c you can clone and update, then update the docs). Without this, if you want to update the guide, you have to recreate everything from scratch again.