[Issue]: Remove duration_minutes field from coaching_sessions and update docs

[RenaudBernier]

Branch Name

fix/72-remove-duration_minutes

Task

Remove duration_minutes from the coaching_sessions table. Right now, when a service is booked, it just copies the duration_minutes from the services table. We should keep 1 source of truth.
Do all necessary refactors.

Acceptance Criteria

• I can create and edit private lessons from the admin dashboard

Checklist

• I have checked for duplicate issues.
• I have assigned the issue to the correct project board.
• I have tagged the issue properly.

Code of Conduct

• I agree to follow this project's Code of Conduct