fix: jwt auth tutorial

[dhmlau]

From the feedback we got in loopbackio/loopback.io#911, I think it would be good if we simply the tutorial. Now that we have the @loopback/authentication-jwt extension, I think it's a good time for us to do it now. While learning how to use the extension, I've created this article (https://medium.com/@MobileDiana/add-jwt-authentication-in-your-loopback-4-application-325137642c2c) and this repo: https://github.com/dhmlau/loopback4-example-todo-jwt.

My plan is:

• preserve the original authentication tutorial because it has lots of good content and explanation in it: See PR fix: refactor auth tutorial loopback4-example-shopping#692
• use this PR for the new JWT tutorial
• (optional) add an example based on https://github.com/dhmlau/loopback4-example-todo-jwt, if we don't have something similar.

@jannyHou @deepakrkris, what do you think? Do we have an example using the new jwt extension?

Checklist

👉 Read and sign the CLA (Contributor License Agreement) 👈

• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• API Documentation in code was updated
• Documentation in /docs/site was updated
• Affected artifact templates in packages/cli were updated
• Affected example projects in examples/* were updated

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[jannyHou]

🙇 Thank you @dhmlau. The plan and the new doc is awesome, every part looks good to me. I think you can turn this PR to be a formal one.

[dhmlau]

@jannyHou, thanks. Let me clean it up a bit before making it available for review.

[jannyHou]

A few comments for the code changes after creating the spec enhancer. Verified the steps with todo-jwt example, all are good 👍

[jannyHou]

👍