docs/setup-{guide,example}: refresh the whole guide

- add many motivation, helpful comments and important details
- improve formatting through use of more native sphinx/rst elements, like
  the csv-table for DNS records
- clarify the basic requirements
- use dig for uncached DNS propagation checks against an authoritative
  nameserver
- explain the basic feature set of the setup example
- adjust DNS TTLs; 1h is a common duration in modern setups and does not
  hurt caching much
- remove mention of the announce mailinglist, users can just expect
  releases to be ready around branch-off

docs/setup-example.nix

@@ -5,10 +5,15 @@
 {
   imports = [
     (builtins.fetchTarball {
-      # Pick a release version you are interested in and set its hash, e.g.
+      # This is a quick and dirty way to import a NixOS mailserver release. What
+      # you should do long-term is use a proper dependency pinning tool like npins
+      # or flakes.
+
+      # URL to the tarball for the release matching your NixOS release
       url = "https://gitlab.com/simple-nixos-mailserver/nixos-mailserver/-/archive/nixos-25.11/nixos-mailserver-nixos-25.11.tar.gz";
-      # To get the sha256 of the nixos-mailserver tarball, we can use the nix-prefetch-url command:
-      # release="nixos-25.11"; nix-prefetch-url "https://gitlab.com/simple-nixos-mailserver/nixos-mailserver/-/archive/${release}/nixos-mailserver-${release}.tar.gz" --unpack
+
+      # Hash of the unpacked tarball, run the following command to retrieve it
+      # release="nixos-25.11" nix-prefetch-url "https://gitlab.com/simple-nixos-mailserver/nixos-mailserver/-/archive/${release}/nixos-mailserver-${release}.tar.gz" --unpack
       sha256 = "0000000000000000000000000000000000000000000000000000";
     })
   ];
@@ -30,18 +35,22 @@
     fqdn = "mail.example.com";
     domains = [ "example.com" ];
 
-    # reference an existing ACME configuration
+    # Reference the existing ACME configuration created by nginx
     x509.useACMEHost = config.mailserver.fqdn;
 
     # A list of all login accounts. To create the password hashes, use
     # nix-shell -p mkpasswd --run 'mkpasswd -s'
     loginAccounts = {
       "user1@example.com" = {
+        # Reads the password hash from a file on the server
         hashedPasswordFile = "/a/file/containing/a/hashed/password";
+
+        # Additional addresses delivered to this mailbox
         aliases = [ "postmaster@example.com" ];
       };
       "user2@example.com" = {
-        # ...
+        # Provides the password hash inline
+        hashedPassword = "$y$j9T$JqqefR6flaaJBRjD4KVZc1$QM6h4Spr5.yn/FuIT.ydTV22daEbiVd8ZprV/POtPgB";
       };
     };
   };