[migration/auth] Create a guide to describe how to migrate LB3 User based authentication to LB4

[raymondfeng]

Create a guide to describe how to migrate LB3 built-in User/AccessToken model based authentication

Acceptance criteria

• Describe usage of LB4 datasource and repository for the user/access token data access
• Migrate model definitions from LB3 User/AccessToken to LB4
• Create a login controller to perform authentication
• Create a token service to generate access tokens
• Create an authentication strategy to validate access tokens
• Update https://github.com/strongloop/loopback-next/blob/auth-migration/docs/site/migration/auth/built-in.md

[dhmlau]

@bajtos @jannyHou, I'd like to continue the discussion from #4571 (comment) of having a reusable extension/component for the JWT authentication. If that's what we're going to create, should we get that component created first before working on this docs?

cc @raymondfeng

[jannyHou]

@dhmlau Yes. Moving here after close #4753

[jannyHou]

Will address the comment in #4706 (comment)

[jannyHou]

closed as PRs merged. 🎉