[New content] Mail services - how to integrate postfix and dovecot

## The task

Although we have individual pages on how to install and configure [Dovecot](https://documentation.ubuntu.com/server/how-to/mail-services/install-dovecot/) and [Postfix](https://documentation.ubuntu.com/server/how-to/mail-services/install-postfix/), they are commonly used together and we are missing a page on how to integrate them.

## Details

It would be great to have a "how to integrate Dovecot and Postfix" page in the documentation. Such a page should live in the same "mail services" topic of the how-to section. It should cross link to both the Dovecot and Postfix pages, and those pages should each receive a short note pointing to the new page.

It should conform to the Diataxis ["how-to guides" guidelines](https://diataxis.fr/how-to-guides/) in approach. There is also a ["how-to guides" template](https://github.com/canonical/ubuntu-server-documentation/blob/main/doc-templates/template-how-to.md) in our contributing section that may be helpful.

Thanks in advance!