Document how to OAUTH2_REDIRECT works and its relation with the integrations

[rafaelromcar-parabol]

Issue - Enhancement

It's unclear how OAUTH2_REDIRECT works, what is a valid value for it and for what it's used. We need some documentation either in a readme or in the .env.example. Maybe both.

Acceptance Criteria (optional)

Users can:

• Understand what OAUTH2_REDIRECT is for and be able to set that correctly reading the documentation.

Estimated effort: 11 points (see CONTRIBUTING.md)

[jordanh]

Related to #10452