docs: centralize Markdown developer manual

docs/src/index.md

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+# Kaptan Developer Manual
+
+Kaptan is an embedded Rust flight-controller project. This manual is the
+single source of project documentation: board support, safety boundaries,
+simulation, development workflow, and design references live here as
+Markdown.
+
+The checked-in firmware is telemetry-only. It does not expose physical motor
+authority, and no document in this site should be read as a physical-flight
+release claim.
+
+## Start here
+
+- New contributors should read [Getting started](getting-started.md).
+- Board work belongs under [Boards](boards/index.md).
+- The currently supported closed-loop simulation is [Host SITL](sitl.md).
+- Before submitting changes, follow [Development and verification](development.md).
+
+## Documentation conventions
+
+Documentation is centralized in `docs/src/`; source directories do not carry
+README files. Keep pages in Markdown, add each page to `SUMMARY.md`, and place
+commands beside the assumptions and safety boundary they depend on.
+
+`TODO(configuration)` marks a required configuration contract that has not
+been implemented. It is not permission for an operator to substitute runtime
+values or bypass an immutable build-time safety decision.