Create bluetooth_hid.md

pages/bluetooth_hid.md

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+# Bluetooth HID
+Pi-KVM is able to emulate a Bluetooth keyboard & mouse.
+This is not the main case of using Pi-KVM since you still need it to pair with a remote host, but can be used for something like mobile KVM.
+
+:exclamation: Using Bluetooth HID requires additional configuration of the operating system. For v2, this means losing the UART port,
+since it will be used by Bluetooth. Also, Bluetooth operation was tested only on RPi4 and v2 platform. Other boards may require different system service settings.
+
+:exclamation: Bluetooth mouse can work only in [relative mode](mouse.md). The reason is that many Bluetooth host drivers do not correctly implement HID descriptors.
+
+### Configuring
+1. Switch filesystem to RW-mode, perform update and install some packages:
+    ```
+    # rw
+    # pacman -Syu
+    # pacman -S bluez bluez-utils raspberrypi-bluetooth
+    ```
+2. Edit `/boot/config.txt` and comment these lines:
+    ```
+    #enable_uart=1
+    #dtoverlay=disable-bt
+    ```
+3. Create an empty directory `/var/lib/bluetooth` and add mountpoint to `/etc/fstab`:
+    ```
+    # mkdir /var/lib/bluetooth
+    # echo 'tmpfs /var/lib/bluetooth tmpfs nodev,nosuid,mode=0755 0 0' >> /etc/fstab
+    ```
+4. Override and enable the services:
+    ```
+    # mkdir /etc/systemd/system/bluetooth.service.d
+    # cat << EOF > /etc/systemd/system/bluetooth.service.d/override.conf
+    [Service]
+    ExecStart=
+    ExecStart=/usr/lib/bluetooth/bluetoothd --noplugin=*
+    EOF
+    # systemctl enable bluetooth
+    # systemctl enable raspberrypi-btuart
+    ```
+5. Override `kvmd` service:
+    ```
+    # mkdir /etc/systemd/system/kvmd.service.d
+    # cat << EOF > /etc/systemd/system/bluetooth.service.d/override.conf
+    [Service]
+    AmbientCapabilities=CAP_NET_RAW CAP_NET_BIND_SERVICE CAP_SYS_ADMIN CAP_SETUID CAP_SETGID CAP_CHOWN
+    CapabilityBoundingSet=CAP_NET_RAW CAP_NET_BIND_SERVICE CAP_SYS_ADMIN CAP_SETUID CAP_SETGID CAP_CHOWN
+    EOF
+    ```