[FC-41933] update the mailserver docs to show an example nix configuration

[PhilTaken]

@flyingcircusio/release-managers

Internal ticket: FC-41933

Update the documentation for the mailserver role as well as fixing capitalization for Rspamd.

Release process

• Created changelog entry using ./changelog.sh

PR release workflow (internal)

• PR has internal ticket
• internal issue ID (PL-…) part of branch name
• internal issue ID mentioned in PR description text
• ticket is on Platform agile board
• ticket state set to Pull request ready
• if ticket is more urgent than within the next few days, directly contact a member of the Platform team

Design notes

• Provide a feature toggle if the change might need to be adjusted/reverted quickly depending on context. Consider whether the default should be on or off. Example: rate limiting.
• All customer-facing features and (NixOS) options need to be discoverable from documentation. Add or update relevant documentation such that hosted and guided customers can understand it as well.

Security implications

• Security requirements defined? (WHERE)
• Security requirements tested? (EVIDENCE)

[osnyx]

I'd like to get confirmation on whether we intend to deprecate json config for maps. Still merging this for now, as existing json config is not broken by just changing the docs.

@zagy @PhilTaken

[osnyx]

rebased onto fc-24.05-dev, hoping this fixes the gitlab test.