Document custom email configuration things

[aldy505]

Problem Statement

Sentry provides 4 mail backend: smtp, dummy, preview (and 1 other thing that I forgot just now). If people are using custom SMTP outgoing output, they should know what to expect (what to configure, providing some prominent/common providers like Sendgrid SMTP or AWS SES SMTP or something) and what to do after changing values on config.yml file. The docs does not cover this as of now.

Discord thread link: https://discord.com/channels/621778831602221064/796028405833007104/1231882692271013898

Solution Brainstorm

No response