Add bluetooth pairing UI README.md documentation.

Fixed: b/303135850
Test: None
Change-Id: I1081ec5a3f93f896533770bafc69089572fe4fb6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4985365
Reviewed-by: Gordon Seto <gordonseto@google.com>
Reviewed-by: Theo Johnson-kanu <tjohnsonkanu@google.com>
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1216423}

ash/webui/common/resources/bluetooth/README.md

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+# ChromeOS Bluetooth Pairing UI
+
+This directory contains Bluetooth pairing UI polymer elements used to display
+information about available Bluetooth devices that can be paired, and UI that
+the user interacts with to pair with a Bluetooth device of their choosing.
+The dialog is either shown within Settings UI, a standalone dialog in sign-in
+screen and OOBE.
+
+Underneath the hood, the elements use the [CrosBluetoothConfig mojo API](https://source.chromium.org/chromium/chromium/src/+/main:chromeos/ash/services/Bluetooth_config/public/mojom/cros_Bluetooth_config.mojom;l=1;bpv=1;bpt=0;drc=321047b607bc69f5d6dce6e47319d0c198d0616e)
+to fetch metadata about available Bluetooth devices to pair with, and to
+actually pair with Bluetooth devices.
+
+## BluetoothBasePage
+Base template with elements common to all Bluetooth UI sub-pages.
+
+## BluetoothBatteryIconPercentage
+View displaying a dynamically colored/sized battery icon and corresponding
+battery percentage string for a given device and battery type.
+
+## BluetoothDeviceBatteryInfo
+View displaying Bluetooth device battery info. Decides whether to show multiple
+battery icon percentages (if the Bluetooth device has multiple associated
+batteries, like wireless earbuds for example) or a single battery icon
+percentage (like a single Bluetooth speaker for example).
+
+## BluetoothIcon
+UI element used to display Bluetooth device icon. Decides whether to show
+system Bluetooth icons depending on the type of device, or the default
+device image if there is an available image url associated to the device.
+
+## BluetoothMetricsUtils
+Used by other components in this directory to record Bluetooth metrics.
+
+## BlueoothPairingConfirmCodePage
+Bluetooth page that displays UI elements for when authentication via
+confirm passkey is required during Bluetooth device pairing.
+
+## BluetoothPairingDeviceItem
+Container used to display information about a single Bluetooth device.
+
+## BluetoothPairingDeviceSelectionPage
+Bluetooth page that displays a list of discovered Bluetooth devices
+and initiate pairing to a device.
+
+## BluetoothPairingEnterCodePage
+Bluetooth page that displays UI elements for when authentication via
+display passkey or PIN is required during Bluetooth device pairing.
+
+## BluetoothPairingRequestCodePage
+Bluetooth page that displays UI elements for when authentication via PIN
+or PASSKEY is required during Bluetooth device pairing.
+
+## BluetoothPairingUi
+Root UI element for Bluetooth pairing dialog. Contains all the Bluetooth
+pairing pages and decides which one to display.
+
+## BluetoothSpinnerPage
+Bluetooth page displayed when a pairing is in progress. Displays a
+pinwheel.
+
+## BluetoothTypes
+Contains enums that are used to describe the type and state of the
+Bluetooth device.
+
+## BluetoothUtils
+Contains utility functions to easily fetch metadata about a
+Bluetooth device.
+
+## CrosBluetoothConfig
+Wrapper for CrosBluetoothConfig that provides the ability to inject
+a fake CrosBluetoothConfig implementation for tests.