docs: add Arch Linux / CachyOS install notes for AIC8800D80

Add INSTALL_ARCH.md covering installation on Arch-based distributions,
validated on a Lukton DQ-211 (AIC8800D80, Pandora 88M80 clone) under
CachyOS with kernels 7.2.0-1-cachyos and 6.18.42-1-cachyos-lts.

The notes cover the AUR/DKMS path, the manual build path, and two gotchas
that are not documented for these distros:

- the AUR package ships aic.rules but not /etc/usb_modeswitch.d/1111:1111,
  so the mode switch silently does nothing without it;
- the DKMS modules live under updates/dkms/ and need depmod -a before
  modprobe aic8800_fdrv can find them.

Also document that aic_load_fw already carries the usb:vA69Cp8D80d* alias,
so no modules-load.d entry is required, plus NetworkManager route-metric
notes for preferring the dongle over the onboard radio.

README: link the new notes from the Installation Steps section.

Co-authored-by: CommandCodeBot <noreply@commandcode.ai>
(cherry picked from commit 68498ff48c)

README.md

@@ -96,6 +96,11 @@ For OpenWrt SDK/buildroot builds, see [OpenWrt build notes](OPENWRT.md).
 OpenWrt can use a newer cfg80211 backport than its target kernel; these builds
 must pass the wireless stack version separately.
 
+For Arch-based distributions (Arch, CachyOS, …), see
+[Arch Linux / CachyOS install notes](INSTALL_ARCH.md) — including the AUR/DKMS
+path, the `/etc/usb_modeswitch.d/1111:1111` file the AUR package does not ship,
+and the `depmod` step needed after DKMS installation.
+
 #### Method 1: [Quick Installation](INSTALL_SCRIPT.md) (Recommended)
 
 #### Method 2: Manual Installation