No docs on STM32Cube builder options

[maxgerhardt]

The current documentation at https://docs.platformio.org/en/latest/frameworks/stm32cube.html lacks all the possible configuration values exposed by the stm32cube.py builder script.

• board_build.stm32cube.custom_config_header = yes/no
• board_build.stm32cube.disable_embedded_libs = yes/no
• meaning of board_build.stm32cube.variant
• board_build.stm32cube.custom_system_setup = yes/no

The documentation should fully document the capabilities of the builder script.