docs: shuffle files to have the same logical hierarchy as on the generated doc

Also:
 - rename some files to remove redundant information from the filenames
 - rename many labels to match the filename and its place in the hierarchy
 - move lua_api under the scripting section

docs/rst/scripting/lua_api/lua_spa_device_api.rst

@@ -0,0 +1,21 @@
+ .. _lua_spa_device_api:
+
+Spa Device
+==========
+
+.. function:: SpaDevice.get_managed_object(self, id)
+
+   Binds :c:func:`wp_spa_device_get_managed_object`
+
+   :param self: the spa device
+   :param integer id: the object id
+   :returns: the managed object or nil
+
+.. function:: SpaDevice.store_managed_object(self, id, object)
+
+   Binds :c:func:`wp_spa_device_store_managed_object`
+
+   :param self: the spa device
+   :param integer id: the object id
+   :param GObject object: a GObject to store or nil to remove the existing
+                          stored object