Netmaker add-on: pull prebuilt image from Gitea registry

Home Assistant add-on that joins the instance to a Netmaker network via
netclient. Distributed as a repository add-on using a prebuilt arm64 image
(gitea.savant.io/homeassistant/netmaker) to avoid local buildx.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

netmaker/.README.j2

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+# Home Assistant Community App: Netmaker
+
+![Project Stage][project-stage-shield] ![Project Maintenance][maintenance-shield]
+
+Join your Home Assistant instance to a Netmaker WireGuard network.
+
+## About
+
+Netmaker is a platform for creating and managing fast, secure WireGuard
+networks. This app joins your Home Assistant instance to a Netmaker network
+using the Netmaker client, `netclient`.
+
+You provide an enrollment token from your Netmaker dashboard, and the app
+registers your device and keeps the connection running in the background.
+
+{% if channel == "edge" %}
+## WARNING! THIS IS AN EDGE VERSION!
+
+This Home Assistant Apps repository contains edge builds of apps.
+Edge builds apps are based upon the latest development version.
+
+- They may not work at all.
+- They might stop working at any time.
+- They could have a negative impact on your system.
+
+{% endif %}
+{% if channel == "beta" %}
+## WARNING! THIS IS A BETA VERSION!
+
+This Home Assistant Apps repository contains beta releases of apps.
+
+- They might stop working at any time.
+- They could have a negative impact on your system.
+
+{% endif %}
+[maintenance-shield]: https://img.shields.io/maintenance/yes/2026.svg
+[project-stage-shield]: https://img.shields.io/badge/project%20stage-experimental-yellow.svg
+[release-shield]: https://img.shields.io/badge/version-{{ version }}-blue.svg
+[release]: {{ repo }}/tree/{{ version }}