init basic examples and setup

docs/hardware/blue-pill.md

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+# Blue Pill Notes
+
+## Baseline
+
+- Board: STM32F103C8T6 Blue Pill
+- Core: Cortex-M3
+- Default target: `thumbv7m-none-eabi`
+
+## LED
+
+- Most Blue Pill boards wire the onboard LED to `PC13`
+- It is usually active-low
+- `set_low()` usually turns it on
+- `set_high()` usually turns it off
+
+## Button Assumption
+
+- The repo button examples use `PA0`
+- Assumed wiring:
+  - button between `PA0` and `3V3`
+  - firmware enables internal pull-down
+
+## Probe Wiring
+
+- `SWDIO` -> `SWDIO`
+- `SWCLK` -> `SWCLK`
+- `GND` -> `GND`
+- `3V3` -> `3V3` reference