docs: Add guide to showcase handling outputs

Add a guide which highlights how to handle action's outputs by writing
them to the client filesystem and using CUE's integrations to marshal
them as JSON.

Signed-off-by: Marcos Lilljedahl <marcosnils@gmail.com>

docs/tests/core-concepts/client/plans/output_marshal.cue.fragment

@@ -0,0 +1,18 @@
+#SomeAction: {
+	// Nop to make dagger run the action
+	core.#Nop & {}
+	url:       "test.com"
+	deployUrl: "test.com/deploy"
+	logsUrl:   "test.com/logs"
+}
+
+dagger.#Plan & {
+	client: filesystem: "output.json": write: contents: json.Marshal({
+		url:       actions.test.url
+		deployUrl: actions.test.deployUrl
+		logsUrl:   actions.test.logsUrl
+	})
+	actions: {
+		test: #SomeAction & {}
+	}
+}

docs/tests/core-concepts/client/plans/output_simple.cue.fragment

@@ -0,0 +1,15 @@
+dagger.#Plan & {
+	client: filesystem: "output.txt": write: contents: actions.test.export.files["/hello.txt"]
+
+	actions: {
+		_image: alpine.#Build & {
+			packages: bash: {}
+		}
+		test: bash.#Run & {
+			input: _image.output
+			script: contents: "echo Hello World! > /hello.txt"
+			export: files: "/hello.txt": string
+		}
+
+	}
+}