These new docs explain a lot of the current implementation for the BLE
stack, with reference to AN5289 14.1. They also detail the additional
requirements around BLE startup behaviour; specifically, that certain
SYS events must be awaited, and that shci_c2_ble_init should be called.

Since the ble feature is now enabled by default for the docs, the
remaining documentation for the BLE behaviour (as implemented by the
`stm32wb-hci` crate) should be bought in automatically.

Fixes #2599