[Doc] Enhance Movehub doc about connectionless Bluetooth messaging

[BertLindeman]

Describe the issue

1. Doc on MoveHub uses block image of PrimeHub and PrimeHub. as command prefix. See picture(s).
2. Explain that the MoveHub cannot broadcast if connected to another device like a PC. As David suggests in #1810

Suggested improvement

1. Drop the hubname from the functions and create a movehub blocking image. Or maybe this is on purpose, so no movehub image needed to be made?
2. Add some text like Note: The MoveHub cannot broadcast if it is connected to another device, like a PC or tablet.

One picture ...

another one

This is from the 3.5.0 doc.

From https://github.com/pybricks/pybricks-api/blob/v3.5.0/doc/main/hubs/movehub.rst

    .. rubric:: Using connectionless Bluetooth messaging

    .. blockimg:: pybricks_blockBleBroadcast_PrimeHub

    .. automethod:: pybricks.hubs::PrimeHub.ble.broadcast

    .. blockimg:: pybricks_blockBleObserve_PrimeHub

    .. automethod:: pybricks.hubs::PrimeHub.ble.observe

I could make a PR for this, but there is no MoveHub blockimg. Assume there needs to be made a new pybricks.hubs::MoveHub.ble.broadcast. And no idea how and where to make it.

ls -1 pybricks_blockBleBroadcast_*Hub.svg
pybricks_blockBleBroadcast_CityHub.svg
pybricks_blockBleBroadcast_EssentialHub.svg
pybricks_blockBleBroadcast_PrimeHub.svg
pybricks_blockBleBroadcast_TechnicHub.svg

Bert