Update benefits.md with EAP registration code info and directions

[Mag-Sul]

Added registration code info and steps to register for EAP website.

Changes proposed in this pull request:

• Link to GSA InSite page which lists the EAP registration code
• Directions on how to access the EAP website (including register if new user)

Security Considerations

I linked to the GSA InSite page which lists the registration code so as not to include the code itself in the public handbook