docs: add docs for call operator

docs/operators/call.md

@@ -0,0 +1,53 @@
+---
+title: "@call"
+---
+
+The **@call** operator is used to reference an `@http` operator. It is useful when you have multiple fields that resolves from the same HTTP endpoint.
+
+```graphql showLineNumbers
+schema {
+  query: Query
+}
+
+type Query {
+  posts: [Post] @http(path: "/posts")
+  user(id: Int!): User @http(path: "/users/{{args.id}}")
+}
+
+type User {
+  id: Int!
+  name: String!
+  username: String!
+  email: String!
+}
+
+type Post {
+  id: Int!
+  userId: Int!
+  title: String!
+  body: String!
+  user: User @call(query: "user", args: [{key: "id", value: "{{value.userId}}"}])
+}
+```
+
+## query
+
+The name of the field that has the `@http` resolver to be called. It is required.
+
+```graphql showLineNumbers
+type Post {
+  userId: Int!
+  user: User @call(query: "user", args: [{key: "id", value: "{{value.userId}}"}])
+}
+```
+
+## args
+
+The arguments to be passed to the `@http` resolver. It is optional.
+
+```graphql showLineNumbers
+type Post {
+  userId: Int!
+  user: User @call(query: "user", args: [{key: "id", value: "{{value.userId}}"}])
+}
+```