Add STM32G431 and STM32G474 board definitions.

[tzarc]

Description

Adds default board configurations for:

• MCU = STM32G431
• MCU = STM32G474

Added documentation changes to state that F401, F411, G431, G474 are also supported.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).