STM32N6 example boot from flash with fsbl and app

[tarfu]

• Adds a new examples/stm32n6-flashboot/ workspace with a two-stage boot system (FSBL +
  application) for the STM32N6570-DK, using embassy-boot for DFU firmware updates
• FSBL runs from SRAM (loaded by boot ROM from signed external NOR flash image), enables debug
  access, configures clocks, initializes XSPI2 in memory-mapped mode, and boots the application via
  embassy-boot
• Application runs directly from memory-mapped external flash (MX66UW1G45G, 128 MB) at 0x70100400
  and demonstrates mark_booted() for confirming successful firmware updates
• Includes a justfile with recipes for building, signing (STM32 signing tool with header v2.3),
  flashing (probe-rs and STM32_Programmer_CLI fallback), and DFU testing (trigger-swap,
  mark-booted, erase-state)
• Moves STM32N6 boot notes from examples/stm32n6/README.md into the new BOOT_NOTES.md to keep the
  original example README focused

As DFU and all the updating stuff is not implemented yet, it is a draft.

The readme is generated to give a quick overview as the flashboot example is bigger then expected.

Thanks a lot to @jake-taf for the description in #5626