docs: add self-signed example

[bt90]

Borrowed the openssl command from https://aiosmtpd.readthedocs.io/en/latest/smtp.html#enabling-starttls and added an explanation on how to properly mount it in the container.

[YoRyan]

I've opted to add links to the page instead with 562e093. This isn't a setup I'd really want to encourage; self-signed TLS is not much better than plaintext, because of the lack of a certificate authority.

[bt90]

While I agree it's not perfect, it's far better than a plaintext connection. Apart from MITM attacks, the credentials are secure at least during transmission.

[YoRyan]

That's fair. Still, once you have your certificate file, it should be fairly obvious how to provide it to the container with bind-mounts. I don't even think it's necessary to set the ownership to 999:999; I doubt aiosmtpd does any permissions checking like the web servers do.