Setup mailman3 server

We want to migrate to mailman3 as mailman2 is basically unmaintained and requires Python 2 which is EOL.

Because the mailman and mailman3 packages conflict and we don't want to perform a big bang migration, mailman3 must be deployed on a separate server. mailman-web (mailman3's web interface) hasn't been packaged yet, so for now we are using my homebrewed PKGBUILD.

[1] https://gist.github.com/klausenbusk/5982063f95c503754a51ed2fefb8915e

Ref #59 (closed)

---

Migrated lists:

• arch-announce
• arch-devops-private
• arch-events
• arch-wiki-admins

docs/servers.md

@@ -151,6 +151,14 @@ Prometheus, and Grafana server which receives selected performance/metrics from
 
   Online collborative markdwown editor for Arch Linux Staff.
 
+## mailman3.archlinux.org
+
+This server runs mailman3 as mailman2 and mailman3 can't be installed on the same server. The HTTP and LMTP traffic is routed over WireGuard from lists.archlinux.org.
+
+### Services
+
+  - mailman3
+
 ### Services
   - [hedgedoc](https://hedgedoc.org/)