docs: restructure the concepts page

[jannyHou]

connect to #5776

I renamed the concept items to use the singular terms.

fold subsections:

unfold subsections, part 1:

unfold subsections, part 2:

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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[dhmlau]

I think I understand the rationale behind changing the html path name is to make it consistent with the title. If that's the case, can we keep the same html and add the redirect_to? Renaming the html path will cause broken links in other docs or articles (mostly external ones, not part of LB docs). WDYT?

[jannyHou]

@dhmlau 👍 Fair enough, adding.

[dhmlau]

Thanks @jannyHou. Your PR LGTM in general. I have one concern about renaming the html path, please see https://github.com/strongloop/loopback-next/pull/6085/files#r468144288. Thanks.

[raymondfeng]

I start to lean toward getting rid of TypeScript and move Decorators and Mixins to other sections.

• Decorators are used to supply metadata (it seems weird to list specific decorators under TypeScript)
• Mixins are a way to compose LoopBack 4 classes

[jannyHou]

Thanks @raymondfeng fair enough, I moved them into "Core", see the updated doc:

[agnes512]

Let's update https://loopback.io/doc/en/lb4/index.html#how-is-our-documentation-organized

[agnes512]

Are we changing the order in this PR?

[jannyHou]

@agnes512 Thanks for the suggestion in #6085 (comment) good catch, and yes the order is changed as well.

[agnes512]

[bajtos]

Nice improvement! ❤️

[bajtos]

+1 to use singular form in permalink and add redirect_from to preserve existing links 👍🏻