docs: centralize Markdown developer manual

docs/src/getting-started.md

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+# Getting started
+
+## Prerequisites
+
+Use the repository toolchain, Rust 1.90. The workspace declares the embedded
+F7 targets and `llvm-tools-preview` in `rust-toolchain.toml`.
+
+For host development, run:
+
+```sh
+cargo test --workspace --offline
+cargo clippy --workspace --all-targets --offline --exclude kaptan-geprc-f722-v2 --exclude kaptan-kakute-f7-mini-v3 -- -D warnings
+```
+
+Bare-metal board packages are not host-test targets. Check them with the
+target-specific commands in [Development and verification](development.md).
+
+## Repository layout
+
+| Path | Responsibility |
+| --- | --- |
+| `boards/` | Immutable board declarations and linker layouts. |
+| `platforms/stm32f7/` | Reviewed F722 RTIC platform and peripheral ownership. |
+| `libs/` | Typed math, estimation, control, safety, protocol, and driver crates. |
+| `tools/sitl/` | Host-only closed-loop Gazebo simulation. |
+| `docs/` | This Markdown documentation site. |
+
+Do not add board-local READMEs. Add a page here instead, then link it from
+`SUMMARY.md`.
+
+## Build the documentation website
+
+The manual is an [mdBook](https://rust-lang.github.io/mdBook/) project. Build
+it locally with a compatible `mdbook` binary:
+
+```sh
+make -C docs build
+```
+
+The generated static site is written to `target/docs/`. The GitHub Pages
+workflow builds that same output from `main`; enable **GitHub Actions** as the
+repository's Pages publishing source once, then deployments are automatic.