Update "Step1: Onboarding"

[hugo-codesfine]

This backlog content can be found here: Migracode-Barcelona/Module-Onboarding#1

• Rewrite the content based on the process we have already decided and included in the Success part here
• Leave the steps on how to create a new issue and the subsequent, but change the content to point out to our airtable form
• Remove step 4. Apply to enroll as a trainee