Add STM32G0 SC8815 + SW2303 external-FB example

[IvanLi-CN]

Summary

• add a new examples/stm32g0-sc8815-sw2303 async example that keeps VBATS tied to VBAT and drives SC8815 in OTG mode with SW2303 telemetry
• wire the example into the top-level Makefile for one-step build/run commands
• document hardware expectations (VBATS connection, 4S defaults, resistor divider) in the example README

Testing

• cargo fmt
• cargo check --manifest-path examples/stm32g0-sc8815-sw2303/Cargo.toml --target thumbv6m-none-eabi
• lefthook pre-commit (fmt + clippy)