docs(desec): document the free deSEC domain option

Mention and recommend the built-in deSEC free domain registration across the
readme, reverse-proxy and local-instance guides, and add a deSEC step to the
new-instance QA checklist.

Co-Authored-By: szaimen <42591237+szaimen@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Simon L. <szaimen@e.mail.de>

local-instance.md

@@ -2,22 +2,25 @@
 It is possible due to several reasons that you do not want or cannot open Nextcloud to the public internet. Perhaps you were hoping to access AIO directly from an `ip.add.r.ess` (unsupported) or without a valid domain.  However, AIO requires a valid certificate to work correctly. Below is discussed how you can achieve both: Having a valid certificate for Nextcloud and only using it locally.
 
 ### Content
-- [1. Tailscale](#1-tailscale)
-- [2. Pangolin](#2-pangolin)
+- [1. deSEC free domain (recommended)](#1-desec-free-domain-recommended)
+- [2. Tailscale](#2-tailscale)
 - [3. The normal way](#3-the-normal-way)
 - [4. Use the ACME DNS-challenge](#4-use-the-acme-dns-challenge)
 - [5. Use Cloudflare](#5-use-cloudflare)
 - [6. Buy a certificate and use that](#6-buy-a-certificate-and-use-that)
 
-## 1. Tailscale
-This is the recommended way. For a reverse proxy example guide for Tailscale, see this guide by [@Perseus333](https://github.com/Perseus333): https://github.com/nextcloud/all-in-one/discussions/6817
+## 1. deSEC free domain (recommended)
+[deSEC](https://desec.io) offers free dynamic-DNS subdomains under `dedyn.io`. AIO can register an account and a subdomain for you automatically — directly from the domain-entry page of the AIO interface. After registration:
+- The [Caddy](https://github.com/nextcloud/all-in-one/tree/main/community-containers/caddy) community container is enabled automatically as a reverse proxy and handles TLS via Let's Encrypt.
+- The [dnsmasq](https://github.com/nextcloud/all-in-one/tree/main/community-containers/dnsmasq) community container is enabled automatically so that LAN clients resolve your Nextcloud domain to the server's local IP address — no separate Pi-hole or local DNS server required.
+- The mastercontainer keeps the DNS record up to date automatically when your public IP changes.
 
-## 2. Pangolin
-[Pangolin](https://pangolin.net/) is an open-source, WireGuard-based remote access platform similar in concept to Tailscale. It uses the **Newt** connector to create outbound-only encrypted tunnels — no inbound ports need to be opened on your firewall. Pangolin handles TLS automatically, providing a valid certificate for your Nextcloud domain.
+**How to set it up:** Open the AIO interface, expand the **"Don't have a domain? Get a free one from deSEC"** section, enter your email and an optional subdomain slug, and click **Register free domain via deSEC**. See the full documentation at [How to get a free domain via deSEC](https://github.com/nextcloud/all-in-one#how-to-get-a-free-domain-via-desec).
 
-You can use either [Pangolin Cloud](https://app.pangolin.net/) (free tier available) or [self-host your own Pangolin server](https://docs.pangolin.net/self-host/quick-install) on a VPS. For private/local-only access, self-hosting Pangolin on a machine within your local network means that Nextcloud never needs to be exposed to the public internet.
+After registration, follow the [dnsmasq documentation](https://github.com/nextcloud/all-in-one/tree/main/community-containers/dnsmasq) to point your router's DHCP DNS server to the AIO host so that all LAN devices resolve the domain locally.
 
-For the reverse proxy configuration details and a step-by-step setup guide, see the [Pangolin section in the reverse proxy documentation](./reverse-proxy.md#pangolin).
+## 2. Tailscale
+For a reverse proxy example guide for Tailscale, see this guide by [@Perseus333](https://github.com/Perseus333): https://github.com/nextcloud/all-in-one/discussions/6817
 
 ## 3. The normal way
 The normal way is the following: