docs(default_ad_api): add readme

system/default_ad_api/README.md

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+# default_ad_api
+
+This package is a default implementation AD API.
+
+- [autoware state (backward compatibility)](document/autoware-state.md)
+- [fail-safe](document/fail-safe.md)
+- [interface](document/interface.md)
+- [localization](document/localization.md)
+- [motion](document/motion.md)
+- [operation mode](document/operation-mode.md)
+- [routing](document/routing.md)

system/default_ad_api/document/autoware-state.md

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+# Autoware state compatibility
+
+## Overview
+
+Since `/autoware/state` was so widely used, default_ad_api creates it from the states of AD API for backwards compatibility.
+The diagnostic checks that ad_service_state_monitor used to perform have been replaced by component_state_monitor.
+The service `/autoware/shutdown` to change autoware state to finalizing is also supported for compatibility.
+
+![autoware-state-architecture](images/autoware-state-architecture.drawio.svg)
+
+## Conversion
+
+This is the correspondence between AD API states and autoware states.
+The launch state is the data that default_ad_api node holds internally.
+
+![autoware-state-table](images/autoware-state-table.drawio.svg)

system/default_ad_api/document/fail-safe.md

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+# Fail-safe API
+
+## Overview
+
+The fail-safe API simply relays the MRM state. See the [autoware-documentation](https://autowarefoundation.github.io/autoware-documentation/main/design/autoware-interfaces/ad-api/list/api/fail_safe/) for AD API specifications.