Add mail policy to Fair Use section of Deploy docs

### What problem are you trying to solve?

I've seen a number of users puzzled at not being able to send emails directly from Deno Deploy. It would be beneficial to have this policy explicitly detailed in the Fair Use documentation of Deploy.

### Describe the solution you'd like

Update the docs with the fair use regarding sending emails via Deploy.

### Describe alternatives you've considered

N/A

### Documentation, Adoption, Migration Strategy

_No response_