Allow passing custom CA as part of email setup

### Description

```
email:
smtp:
host: mail.example.com
tls:
ca:
- "PEM ENCODED CA CERT"
```

Shouldn't require any code changes, just docs for email setup.

https://stackoverflow.com/a/66868102

Should also document:

```
email:
smtp:
host: mail.example.com
tls:
rejectUnauthorized: false
```

As a workaround.