docs: Add config guide for relaying to and from a private DMS instance

[furstblumier]

Description

This PR adds an example of how to setup a relay-only public instance of postfix to use with a private DMS instance.

Type of change

• Improvement (non-breaking change that does improve existing functionality)
• This change requires a documentation update

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary, I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have added information about changes made in this PR to CHANGELOG.md

[georglauterbach]

LGTM 👍🏼 I would like to wait for @polarathene to review this too, though. Everyone is low on time currently, so it might take a bit. The effort is greatly appreciated, though! :)

[polarathene]

This looks pretty good, thanks for taking the time to contribute it!

Bit low on time for proper review cycle, so hope you don't mind with the direct commits I made. Please view the preview link to let me know what you think of the changes, if anything looks odd/off we can adjust, otherwise this seems good to merge 👍

I've not gone over the referenced discussion where I think I gave some other advice or cited some gotchas/alternatives to have in mind. So the guide could possibly be improved further but linking to the discussion at the top as I've added should be enough for anyone that lands on it 😅 might as well save ourselves some time until that extra information is useful to someone else.

---

Please update the CHANGELOG.md "Added" section to include this at the start of the list:

- **Docs:**
  - A guide for configuring a public server to relay inbound and outbound mail from DMS on a private server ([#3973](https://github.com/docker-mailserver/docker-mailserver/pull/3973))

[georglauterbach]

ping @furstblumier

[furstblumier]

Hi, sorry was pretty busy this week. Thanks a ton for the fixups, the markdown looks wayyyy better like this! :)
As said in my previous reply I'm ok with omitting that line, if that's the only thing that's bothering people.

[polarathene]

Cheers @casperklein for the extra pass with corrections 😁

Thanks @furstblumier for the great contribution! 🎉

[github-actions]

Documentation preview for this PR is ready! 🎉

Built with commit: 6701764