add Sphinx documentation

docs/reference.md

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+# API Reference
+
+Auto-generated from the source docstrings.
+
+## CLI (`nexus-cli`)
+
+```{eval-rst}
+.. automodule:: nexus_sync.cli.__main__
+```
+
+## Client
+
+```{eval-rst}
+.. automodule:: nexus_sync.client.runtime
+
+.. automodule:: nexus_sync.client.execute
+```
+
+## Server
+
+```{eval-rst}
+.. automodule:: nexus_sync.server.app
+
+.. automodule:: nexus_sync.server.store
+
+.. automodule:: nexus_sync.server.sqlalchemy_store
+
+.. automodule:: nexus_sync.server.config
+```
+
+## Common
+
+```{eval-rst}
+.. automodule:: nexus_sync.common.models
+```
+
+## Utilities
+
+```{eval-rst}
+.. automodule:: nexus_sync.i18n
+
+.. automodule:: nexus_sync.utils.log_config
+```