boards: doc: Update information about generating litex_vexriscv SoC
boards: doc: Update information about generating litex_vexriscv SoC

This commit updates information about running ZephyrOS on litex_vexriscv
SoC. It also add information about generating bitstream with
Zephyr on LiteX VexRiscv and LiteX SoC Builder.

Signed-off-by: Mateusz Sierszulski <31fdbe6085304ea873c2e49eb8a8540c25fbf1c6@antmicro.com>
Co-authored-by: Jagoda Szwedyk <7afb9127341d6f4c9aa53fa5bfac1a4ac0ef2a15@antmicro.com>

zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr