Fix broken image embed in documentation

The file previously targeted by the embed has been lost or moved. Experience shows that this is is a common occurrence
with the files hosted by Arduino, and will be especially problematic in cases like this where the file may appear to be
superfluous to the website maintainers due to the lack of any internal references to it, so the only way to ensure the
file will continue to be available is by hosting alongside the documentation content in the repository.

I was able to recover the lost file from the Internet Archive Wayback Machine's 2023-06-18 archive of the previous URL:

https://web.archive.org/web/20230618230604/https://www.arduino.cc/en/uploads/Reference/ble-bulletin-board-model.png

The absolute URL of the file was intentionally used in the markup instead of what would normally be the best practices
of using the relative path. The reason is because the documentation content will be published at
https://reference.arduino.cc/reference/en/libraries/arduinoble/ and the publishing system doesn't have any capability
for handling asset files.

docs/readme.md

@@ -11,7 +11,7 @@ Bluetooth® 4.0 includes both traditional Bluetooth®, now labeled "Bluetooth®
 
 Unlike standard Bluetooth® communication basically based on an asynchronous serial connection (UART) a Bluetooth® LE radio acts like a community bulletin board. The computers that connect to it are like community members that read the bulletin board. Each radio acts as either the bulletin board or the reader. If your radio is a bulletin board (called a peripheral device in Bluetooth® LE parlance) it posts data for all radios in the community to read. If your radio is a reader (called a central device in Blueooth LE terms) it reads from any of the bulletin boards (peripheral devices) that have information about which it cares. You can also think of peripheral devices as the servers in a client-server transaction, because they contain the information that reader radios ask for. Similarly, central devices are the clients of the Bluetooth® LE world because they read information available from the peripherals.
 
-![Communication between central and peripheral devices](https://www.arduino.cc/en/uploads/Reference/ble-bulletin-board-model.png)
+![Communication between central and peripheral devices](https://raw.githubusercontent.com/arduino-libraries/ArduinoBLE/master/docs/assets/ble-bulletin-board-model.png)
 
 Think of a Bluetooth® LE peripheral device as a bulletin board and central devices as viewers of the board. Central devices view the services, get the data, then move on. Each transaction is quick (a few milliseconds), so multiple central devices can get data from one peripheral.