add Sphinx documentation

docs/index.md

@@ -0,0 +1,24 @@
+# nexus-sync
+
+A utility that allows you to manage your computers from a centralized server.
+
+nexus-sync has three parts: a central **server** (FastAPI), a **client** agent
+that runs on each managed machine and executes a whitelist of commands, and
+**nexus-cli**, the operator's command-line tool.
+
+```{toctree}
+:maxdepth: 2
+:caption: Guide
+
+api
+cli
+client
+server
+```
+
+```{toctree}
+:maxdepth: 2
+:caption: Reference
+
+reference
+```