Rewrite "A Freestanding Rust Binary" (#577)

- solve linker errors by compiling for thumbv7em-none-eabihf
- make old section about linker arguments optional content
- use different linker arguments on Windows and macOS to use the `_start` entry point on all OSs
- Add an (optional) section that explains how to set linker arguments in .cargo/config
- Add an (optional) section that discourages the linker arguments approach and recommends the `#[start]` attribute instead
- reorganize section levels