Split README into sphinx doc structured sections (#711)

refactor README.rst and architecture file into sphinx doc project, automatically deploying on main merges and PRs.

* add FAQs from https://chatmail.at/relays landing page

* fix links, and streamline postfix/dovecot mentioning

* add linkcheck to CI, fix several links and streamlihne DKIM section while at it

* some streamlining, rename to "overview"

* ci: upload documentation to chatmail.at/doc/relay

* ci: main should be uploaded when docs.yaml changes

* ci: fix typo

* Update .github/workflows/docs-preview.yaml

Co-authored-by: missytake <missytake@systemli.org>

doc/source/proxy.rst

@@ -0,0 +1,114 @@
+
+Setting up a reverse proxy
+--------------------------
+
+A chatmail relay MTA does not track or depend on the client IP address
+for its operation, so it can be run behind a reverse proxy. This will
+not even affect incoming mail authentication as DKIM only checks the
+cryptographic signature of the message and does not use the IP address
+as the input.
+
+For example, you may want to self-host your chatmail relay and only use
+hosted VPS to provide a public IP address for client connections and
+incoming mail. You can connect chatmail relay to VPS using a tunnel
+protocol such as `WireGuard <https://www.wireguard.com/>`_ and setup a
+reverse proxy on a VPS to forward connections to the chatmail relay over
+the tunnel. You can also setup multiple reverse proxies for your
+chatmail relay in different networks to ensure your relay is reachable
+even when one of the IPs becomes inaccessible due to hosting or routing
+problems.
+
+Note that your chatmail relay still needs to be able to make outgoing
+connections on port 25 to send messages outside.
+
+To setup a reverse proxy (or rather Destination NAT, DNAT) for your
+chatmail relay, put the following configuration in
+``/etc/nftables.conf``:
+
+::
+
+   #!/usr/sbin/nft -f
+
+   flush ruleset
+
+   define wan = eth0
+
+   # Which ports to proxy.
+   #
+   # Note that SSH is not proxied
+   # so it is possible to log into the proxy server
+   # and not the original one.
+   define ports = { smtp, http, https, imap, imaps, submission, submissions }
+
+   # The host we want to proxy to.
+   define ipv4_address = AAA.BBB.CCC.DDD
+   define ipv6_address = [XXX::1]
+
+   table ip nat {
+           chain prerouting {
+                   type nat hook prerouting priority dstnat; policy accept;
+                   iif $wan tcp dport $ports dnat to $ipv4_address
+           }
+
+           chain postrouting {
+                   type nat hook postrouting priority 0;
+
+                   oifname $wan masquerade
+           }
+   }
+
+   table ip6 nat {
+           chain prerouting {
+                   type nat hook prerouting priority dstnat; policy accept;
+                   iif $wan tcp dport $ports dnat to $ipv6_address
+           }
+
+           chain postrouting {
+                   type nat hook postrouting priority 0;
+
+                   oifname $wan masquerade
+           }
+   }
+
+   table inet filter {
+           chain input {
+                   type filter hook input priority filter; policy drop;
+
+                   # Accept ICMP.
+                   # It is especially important to accept ICMPv6 ND messages,
+                   # otherwise IPv6 connectivity breaks.
+                   icmp type { echo-request } accept
+                   icmpv6 type { echo-request, nd-neighbor-solicit, nd-router-advert, nd-neighbor-advert } accept
+
+                   # Allow incoming SSH connections.
+                   tcp dport { ssh } accept
+
+                   ct state established accept
+           }
+           chain forward {
+                   type filter hook forward priority filter; policy drop;
+
+                   ct state established accept
+                   ip daddr $ipv4_address counter accept
+                   ip6 daddr $ipv6_address counter accept
+           }
+           chain output {
+                   type filter hook output priority filter;
+           }
+   }
+
+Run ``systemctl enable nftables.service`` to ensure configuration is
+reloaded when the proxy relay reboots.
+
+Uncomment in ``/etc/sysctl.conf`` the following two lines:
+
+::
+
+   net.ipv4.ip_forward=1
+   net.ipv6.conf.all.forwarding=1
+
+Then reboot the relay or do ``sysctl -p`` and
+``nft -f /etc/nftables.conf``.
+
+Once proxy relay is set up, you can add its IP address to the DNS.
+