Add basic usage to documentation root
[stm]: https://www.st.com/en/microcontrollers-microprocessors/stm32f3-series.html
+ ## Basic Usagee
+
+ ```rust
+ #![no_std]
+ #![no_main]
+
+ use cortex_m::asm;
+ use cortex_m_rt::entry;
+ use panic_halt as _;
+ use stm32f3xx_hal::{self as hal, pac, prelude::*};
+
+ #[entry]
+ fn main() -> ! {
+ let dp = pac::Peripherals::take().unwrap();
+
+ let mut rcc = dp.RCC.constrain();
+ let mut gpioe = dp.GPIOE.split(&mut rcc.ahb);
+
+ let mut led = gpioe
+ .pe13
+ .into_push_pull_output(&mut gpioe.moder, &mut gpioe.otyper);
+
+ loop {
+ led.toggle().unwrap();
+ asm::delay(8_000_000);
+ }
+ }
+ ```
+
## Cargo features
### Target chip selection
