Add gateway node-type documentation

Author: JitpanuM

docs/node/nodes/gateway.md

@@ -0,0 +1,116 @@
+---
+sidebar_label: Gateway
+hide_table_of_contents: true
+---
+
+# Gateway
+
+The `gateway` node-type enable VILLASnode to transform api type. 
+Currently, the transformation of [http](https://en.wikipedia.org/wiki/HTTP) and [gRPC](https://grpc.io/) is supported. 
+
+
+### Limitations
+
+- For protobuf payload that is not [VILLASnode format](../formats/protobuf.md), only simple datatypes are supported. 
+- It only supported unary RPC.
+
+
+## Prerequisites
+
+This node-type requires [gRPC](https://grpc.io/) and [reflection.proto](https://github.com/grpc/grpc/blob/master/src/proto/grpc/reflection/v1alpha/reflection.proto) for gRPC server [reflection](https://grpc.io/docs/guides/reflection/).
+The script `packaging/deps.sh` can be used to automatically download reflection.proto and generate protobuf code.
+Please refer to the [installation document](../installation.md).
+
+## Implementation
+
+The source code of the node-type is available here:
+https://github.com/VILLASframework/node/blob/master/lib/nodes/gateway.cpp
+
+## Configuration {#config}
+
+import ApiSchema from '@theme/ApiSchema';
+
+<ApiSchema id="node" example pointer="#/components/schemas/api" />
+
+## Example
+
+``` url="external/node/etc/examples/nodes/gateway.conf" title="node/etc/examples/nodes/api.conf"
+http = {
+    port = 8080
+}
+
+nodes = {
+    gateway_node = {
+        type = "gateway"
+
+        format = "protobuf"
+        #  API type
+        gateway_type = "gRPC"
+        # Address of remote server
+        address = "localhost:50051"
+    }
+}
+```
+
+## Usage
+
+The usage of this node is similar to [`api`](api.md) node-type.
+The following [`curl`](https://curl.se/) commands are example for calling gRPC method with HTTP api.
+The input of gRPC method can be either by the body of http request or input data from other node(via a [path](../config/paths.md)).
+If the http request body is empty, the input from other node will be used.
+
+Note on http methods
+ - GET should be used only when the input of gRPC method can be empty or ignored.
+ - PUT should be used when the output of gRPC method is in VILLASnode format and the user want to put the data to the path.
+ - POST should be used when the output of gRPC method is not in VILLASnode format or the user not want output data to the path.
+
+
+#### The url for sending a request to this node-type are as following
+```
+http://<address>:<port>/api/v2/gateway/<node name or UUID>/<gRPC package>/<gRPC service>/<gRPC method>
+```
+
+The following `.proto` file will be used as an example
+```
+syntax = "proto3";
+
+import "villas.proto";
+
+package ex_server;
+
+service ex_service {
+    rpc GetData (villas.node.Message) returns (villas.node.Message) {};
+    rpc SetData (villas.node.Message) returns (msg) {};
+    rpc GetDataRef (Reference) returns (villas.node.Message) {};
+}
+
+message Reference {
+    repeated int32 ref = 1;
+}
+
+message msg {
+    string status = 1;
+}
+
+```
+
+#### Call GetData method on gRPC server
+
+```shell
+curl http://localhost:8080/api/v2/gateway/gateway_node/ex_server/ex_service/GetData -XPUT
+```
+Since the PUT method is used, the output from gRPC method will be put to the path.
+
+#### Call SetData method on gRPC server
+
+```shell
+curl http://localhost:8080/api/v2/gateway/gateway_node/ex_server/ex_service/SetData -XPOST
+```
+Since the body is empty the data from the path will be send to gRPC server.
+
+#### Call GetDataRef method on gRPC server
+
+```shell
+curl http://localhost:8080/api/v2/gateway/gateway_node/ex_server/ex_service/GetDataRef -d '{"ref":[0,1,2,3,4]}'
+```
+Since the body is not empty, it will be form into protobuf payload based on the example `.proto` file and send to the gRPC server.