Install: a step-by-step guide, and choose NetworkManager in archinstall

A numbered guide at the top of the Install page, from writing the Arch
ISO to a USB stick to the reboot after installing CoreDesktop.

CoreDesktop now turns off iwd and systemd-networkd when they're enabled
alongside NetworkManager (cf862ec), as archinstall's "Copy ISO network
configuration" leaves them. Tell users to choose "Use NetworkManager"
instead, and that the iwctl connection stays on the USB stick. List the
change on the System page, and mention it for NetworkManager and impala.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

content/system.md

@@ -130,6 +130,7 @@ sections:
       - ["systemd", "A 10-second stop timeout (`/etc/systemd/system.conf.d/` and `/etc/systemd/user.conf.d/10-coredesktop-timeout.conf`), systemd-oomd for apps (`/etc/systemd/user/app.slice.d/10-coredesktop-oomd.conf`), and NetworkManager-wait-online masked"]
       - ["`/etc/security/pam_env.conf`", "A PATH line, so SSH commands find `~/.local/bin` and mise"]
       - ["mDNS", "Avahi is on, nss-mdns is added to `/etc/nsswitch.conf`, and SSH is advertised in `/etc/avahi/services/ssh.service`"]
+      - ["Wi-Fi", "If iwd is turned on alongside NetworkManager (archinstall's **Copy ISO network configuration** does that), iwd and systemd-networkd are turned off, so NetworkManager runs the network from the next restart. Choose your Wi-Fi network once in GNOME's top-right menu after that restart. If you set NetworkManager to use iwd itself (`wifi.backend=iwd`), iwd is left alone"]
       - ["Services", "Avahi, Bluetooth, CUPS, fwupd, GDM, NetworkManager, sshd, Docker, Tailscale, ufw and systemd-oomd are turned on. Your user is added to the docker group"]
       - ["Snapper", "btrfs only: a config for `/` in `/etc/snapper/configs/root`"]
       - ["NVIDIA", "Only if it installs the driver: `/etc/mkinitcpio.conf.d/nvidia.conf` and NVIDIA's suspend services"]