Create a Mail page under Server Configuration

[stevenlinx]

Extending from #923 of Documentation-Issue repo.

The second point recommends mentioning the need for a sendmail binary configured on the system in order for wp_mail() to work.

Currently, the AAH doesn't not have a dedicated page about mail related configuration, but should have.

This page can be created mentioning the above point, along with other mail related information. The other related information can be updated over time in phases.