docs: Add ARCHITECTURE.md with diagram of components

- For starters, this file is just a diagram of components of a
  chatmail server.  In the future, this document can grow into a more
  complete description of the architecture of the server, the
  deployment process, and the design intent behind what is and isn't
  in the code base.
- The name ARCHITECTURE.md is inspired by this article, which also has
  good suggestions about what to put in the file:
  https://matklad.github.io/2021/02/06/ARCHITECTURE.md.html

ARCHITECTURE.md

@@ -0,0 +1,37 @@
+This diagram shows components of the chatmail server:
+
+```mermaid
+graph LR;
+    cmdeploy --> sshd;
+    cron --> expunge;
+    cron --> acmetool;
+    cron --> chatmail-metrics;
+    chatmail-metrics --> /var/www/html;
+    acmetool --> certs;
+    acmetool --> acmetool-redirector;
+    acmetool-redirector --> certs;
+    nginx --> /var/www/html;
+    nginx --> certs;
+    nginx --> newemail.py;
+    nginx --> |465|postfix;
+    nginx --> autoconfig.xml;
+    nginx --> |993|dovecot;
+    autoconfig.xml --> postfix;
+    autoconfig.xml --> dovecot;
+    postfix --> certs;
+    postfix --> /home/vmail/mail;
+    postfix --> |10080,10081|filtermail;
+    postfix --> echobot;
+    postfix --> |doveauth.socket|doveauth;
+    dovecot --> certs;
+    dovecot --> |doveauth.socket|doveauth;
+    dovecot --> /home/vmail/mail;
+    dovecot --> |metadata.socket|chatmail-metadata;
+    doveauth --> /home/vmail/mail;
+    expunge --> /home/vmail/mail;
+    chatmail-metadata --> iroh-relay;
+```
+
+(Arrows in this diagram do not have a specific formal meaning; they
+signify "depends on", or "uses", or "sends data to", or just "relates
+to".)