Rearrange admin API fs heirarchy to match sidebar

I've kept the RST redirect file in-place though

docs/administration/admin_api/version_api.md

@@ -0,0 +1,21 @@
+# Version API
+
+This API returns the running Synapse version and the Python version
+on which Synapse is being run. This is useful when a Synapse instance
+is behind a proxy that does not forward the 'Server' header (which also
+contains Synapse version information).
+
+The api is:
+
+```
+GET /_synapse/admin/v1/server_version
+```
+
+It returns a JSON body like the following:
+
+```json
+{
+    "server_version": "0.99.2rc1 (b=develop, abcdef123)",
+    "python_version": "3.7.8"
+}
+```