Add simple tutorial for getting started with authentication

[joeytwiddle]

Trying to make it easy for a developer to quickly get started with authentication. Related issue: #911

You can read this tutorial partially formatted at:

https://github.com/joeytwiddle/loopback.io/blob/quick-authentication/pages/en/lb4/tutorials/authentication/Quick-Authentication-Tutorial-Session-Tokens.md

[dhmlau]

@joeytwiddle, thanks for the PR. Really appreciate you put together this tutorial!
The source of the LoopBack 4 documentation is actually in the loopback-next repo. We have scripts to copy over the content to the loopback.io repo regularly. Therefore, could you please submit a PR in the loopback-next repo instead?

Here is the corresponding md file:
https://github.com/strongloop/loopback-next/blob/master/docs/site/Loopback-component-authentication.md
And the sidebar is here: https://github.com/strongloop/loopback-next/tree/master/docs/site/sidebars.

[dhmlau]

I'd like to close this PR. Looking forward to your PR over the loopback-next repo. Thanks!