Migration guide for model relations

[bajtos]

Describe how to migrate model relations from LB3 to LB4.

Fix #3948

Checklist

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• 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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[bajtos]

The pull request is ready for review, @raymondfeng @strongloop/loopback-maintainers PTAL.

I found two more errors I'd like to fix outside of this PR, they are not blocking the guide.

1. lb4 repository fails when src/repositories directory does not exist (yet) -- see fix(cli): handle missing target artifact dir #4112

2. The relation controller scaffolded by lb4 relation allows FK property (e.g. categoryId) to be included when creating related models (e.g. POST /categories/{categoryId}/products).

   UPDATE: Never mind, I see that the FK property is marked as optional. It looks like an intentional decision, in which case I am fine with that.

[agnes512]

LGTM 👍 , just some nitpicks

[emonddr]

Great stuff

[jannyHou]

A few nitpicks : ) LGTM

[jannyHou]

Maybe make it more clear like:

Run `lb4 relation` to create the model relations according to the relation definitions defined in lb3 model json files.

so that people understand where to find the relation definitions.
And we can link to the generator doc as well: https://loopback.io/doc/en/lb4/Relation-generator.html

[bajtos]

according to the relation definitions defined in lb3 model json files.

[bajtos]

according to the relation definitions defined in lb3 model json files.

I am concerned that this is not entirely correct. Relations can be defined from JS code too, e.g. Product.belongsTo(Category).

I am not sure how to best incorporate this information into the migration guide. Let's leave it out of scope of this pull request. If you have some ideas how to improve the current text, then can you please open a follow-up pull request yourself? 

And we can link to the generator doc as well: https://loopback.io/doc/en/lb4/Relation-generator.html

Great idea 👍

[nabdelgadir]

👍