Update mentor GSoC guidelines

Update mentor guidelines with a checklist containing:

• Explicit CPT check with student
• Student "summer" schedule
• Student other commitments
• Student vacations

Mentors are expected to:

• respond within 48hours (repeat the google requirement for mentors)

Explicitly specify in the mentor guidelines that:

• mentors need to agree on when PR are reviewed
• in the case of multiple mentors, let the student know who will be on the review