partially document admin API

docs/ADMIN_API.md

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+# Libsql-server admin API documentation
+
+This document describes the admin API endpoints.
+
+The admin API is used to manage namespaces on a `sqld` instance. Namespaces are isolated database within a same sqld instance.
+
+To enable the admin API, and manage namespaces, two extra flags need to be passed to `sqld`:
+- `--admin-listen-addr <addr>:<port>`: the address and port on which the admin API should listen. It must be different from the user API listen address (whi defaults to port 8080).
+- `--enable-namespaces`: enable namespaces for the instance. By default namespaces are disabled.
+
+## Routes
+
+```
+POST /v1/namespaces/:namespace/create
+```
+Create a namespace named `:namespace`.
+body:
+```json
+{
+    "dump_url"?: string,
+}
+```
+
+```
+DELETE /v1/namespaces/:namespace
+```
+
+Delete the namespace named `:namespace`.
+
+```
+POST /v1/namespaces/:namespace/fork/:to
+```
+Fork `:namespace` into new namespace `:to`