doc: Add installation instructions for ArchLinux

docs/install/archlinux.md

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+These are instructions for pacman based distributions, like ArchLinux. The package is available from the [AUR](https://aur.archlinux.org/packages/tandoor-recipes-git) or from [GitHub](https://github.com/jdecourval/tandoor-recipes-pkgbuild).
+
+## Features
+- systemd integration.
+- Provide configuration for Nginx.
+- Use socket activation.
+- Use a non-root user.
+- Apply migrations automatically.
+
+## Installation
+1. Clone the package, build and install with makepkg:
+```shell
+git clone https://aur.archlinux.org/tandoor-recipes-git.git
+cd tandoor-recipes-git
+makepkg -si
+```
+or use your favourite AUR helper.
+
+2. Setup a PostgreSQL database and user, as explained here: https://docs.tandoor.dev/install/manual/#setup-postgresql
+
+3. Configure the service in `/etc/tandoor/tandoor.conf`.
+
+4. Reinstall the package, or follow [the official instructions](https://docs.tandoor.dev/install/manual/#initialize-the-application) to have tandoor creates its DB tables.
+
+5. Optionally configure a reverse proxy. A configuration for Nginx is provided, but you can Traefik, Apache, etc..
+Edit `/etc/nginx/sites-available/tandoor.conf`. You may want to use another `server_name`, or configure TLS. Then:
+```shell
+cd /etc/nginx/sites-enabled
+ln -s ../sites-available/tandoor.conf
+systemctl restart nginx
+```
+
+6. Enable the service
+```shell
+systemctl enable --now tandoor
+```
+
+## Upgrade
+```shell
+cd tandoor-recipes-git
+git pull
+makepkg -sif
+```
+Or use your favourite AUR helper.
+You shouldn't need to do anything else. This package applies migration automatically. If PostgreSQL has been updated to a new major version, you may need to [run pg_upgrade](https://wiki.archlinux.org/title/PostgreSQL#pg_upgrade).
+
+## Help
+This package is non-official. Issues should be posted to https://github.com/jdecourval/tandoor-recipes-pkgbuild or https://aur.archlinux.org/packages/tandoor-recipes-git.