add SMTP docs

[rafaeleyng]

Hello, team!

There are quite a few issues of people not knowing how to setup the SMTP config (#44, #47, #76, #82).

I suggest we add a bit of documentation about that.

Thanks!

[Philipose]

Much needed!
A lot of my anxiety (Besides finding the instructions somewhere random place), I did not realize my cloud provider was blocking SMTP traffic to fight spam coming from their servers. Would it makes sense to just that if you are experiencing errors to check if your cloud provider blocks SMTP by default?

[br4yd]

In my case smtps://... didn't work. I had to use smtp:// with 587 as the port. Before that I got internal server error after ages of loading screen and the following error:

{"level":50,"time":1731187561636,"pid":6,"hostname":"<hostname>","name":"xrpc-server","err":{"type":"Error","message":"Connection timeout","stack":"Error: Connection timeout\n    at SMTPConnection._formatError (/app/node_modules/.pnpm/nodemailer@6.9.9/node_modules/nodemailer/lib/smtp-connection/index.js:790:19)\n    at SMTPConnection._onError (/app/node_modules/.pnpm/nodemailer@6.9.9/node_modules/nodemailer/lib/smtp-connection/index.js:776:20)\n    at Timeout.<anonymous> (/app/node_modules/.pnpm/nodemailer@6.9.9/node_modules/nodemailer/lib/smtp-connection/index.js:235:22)\n    at listOnTimeout (node:internal/timers:573:17)\n    at process.processTimers (node:internal/timers:514:7)","code":"ETIMEDOUT","command":"CONN"},"msg":"unhandled exception in xrpc method com.atproto.server.requestEmailConfirmation"}

This still lead to my mail arriving in spam folder even though I have verified DKIM and SPF enties, as well as verified DMARC at resend but at least it arrived.