Migrate apps using loopback-component-passport with LB3 Composition

[deepakrkris]

This is a follow-up for #3718 and #3922.

Scoped from #3958

Write content for docs/site/migration/auth/passport.md, explain how to migrate LB3 applications using loopback-component-passport to LB4.

For short to medium term, while users are running both LB3 and LB4 app at the same time, we can show them how to use Passport as Express middleware, see #3947.

Acceptance criteria

Migration guide describing manual steps for the following scenarios:

• Both LB3 and LB4 applications running in the same process, we want to use a single passport configuration to secure both LB3 and LB4 routes.