* Use rp2040-hal in all example (possibly through their bsp)
Some of the examples were using the cortex_m_rt::entry method which
misses the device specific spinlock re-initialisation.
This commits makes the usage more consistent by using rp2040_hal exported
macro as the only `entry` method used across examples.
* Add basic multicore fifo example
* Add documentation for multicore
* Send system_clock frequency to core1 over FIFO in example
* Add Stack::new() to HAL. Use Stack::new() in example