(from Discourse) add Dovecot SASL to Exim docs

### Page URL

https://documentation.ubuntu.com/server/how-to/mail-services/install-exim4/

### Select an option

- [ ] I found what I was looking for
- [ ] I couldn't find what I was looking for
- [ ] I found the information, but it was incorrect
- [ ] I found the information, but it was incomplete
- [ ] I found the information, but it was confusing
- [x] I encountered a technical issue (e.g., broken link, image not loading)

### Issue Description

Reported [via Discourse](https://discourse.ubuntu.com/t/install-and-configure-exim4/11530/4?u=sally-makin):

It might be helpful to add Dovecot SASL to the Exim SMTP authentication documentation. It would be especially useful for people with Dovecot installed already, negating the need for saslauthd to be installed and configured.

The default Exim configuration already has commented lines for it:
```
# dovecot_plain_server:
# driver = dovecot
# public_name = PLAIN
# server_socket = /var/spool/exim4/dovecot.auth-client
# server_set_id = $auth1
# .ifndef AUTH_SERVER_ALLOW_NOTLS_PASSWORDS
# server_advertise_condition = ${if eq{$tls_in_cipher}{}{}{*}}
# .endif
```

Start with the Exim installation page at https://documentation.ubuntu.com/server/how-to/mail-services/install-exim4/ and review the linked Discourse report. Use the commented Dovecot SASL configuration shown in the issue as context; done means the page documents this option clearly for users who already have Dovecot installed.

Written by the indexing model from the issue text.