docs(php) - sending emails

[sc-zenokerr]

Fixes #1085

This PR adds some basic information about sending emails from PHP, and an example using Brevo.

[EtienneM]

Did you tested this code? If yes and you confirm it's working, then LGTM :)

[Frzk]

I'm not sure whether documenting a specific framework is really what we want to do (because maintenance, etc.)

Also, we should probably advise to use Composer to install the dependency, like so:

composer require phpmailer/phpmailer

[sc-zenokerr]

@Frzk I actually agree.. do you think it would be ok to remove the sample?
It is difficult to test in anycase (without us having an account with Brevo)

[aurelien-reeves-scalingo]

I agree with @Frzk

Why Brevo specifically?
Specially as this is just regular SMTP here. There is no need to limit the case to Brevo IMO