Add back references to passport adapter in authentication docs

[emonddr]

Description / Steps to reproduce / Feature proposal

Not that PR [LABS] feat: adapter that wraps strategy #3056 has landed, let's add back passport adapter references in the authentication documentation where applicable.

Acceptance Criteria

• The Authentication documentation should have a link to passport-adapter documentation
• The @loopback/authentication/README.MD should have a link to the passport-adapter documentation.

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[dhmlau]

@emonddr , could you please add the acceptance criteria? Thanks.

[jannyHou]

PR created for it: #3225