TEMPLATES: Add multi-index templates (#13490)

One that routes and one that fuses

---------

Co-authored-by: Erick Friis <erick@langchain.dev>

templates/rag-multi-index-router/README.md

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+# RAG with Multiple Indexes (Routing)
+
+A QA application that routes between different domain-specific retrievers given a user question.
+
+## Environment Setup
+
+This application queries PubMed, ArXiv, Wikipedia, and [Kay AI](https://www.kay.ai) (for SEC filings).
+
+You will need to create a free Kay AI account and [get your API key here](https://www.kay.ai). 
+Then set environment variable:
+
+```bash
+export KAY_API_KEY="<YOUR_API_KEY>"
+```
+
+## Usage
+
+To use this package, you should first have the LangChain CLI installed:
+
+```shell
+pip install -U langchain-cli
+```
+
+To create a new LangChain project and install this as the only package, you can do:
+
+```shell
+langchain app new my-app --package rag-multi-index-router
+```
+
+If you want to add this to an existing project, you can just run:
+
+```shell
+langchain app add rag-multi-index-router
+```
+
+And add the following code to your `server.py` file:
+```python
+from rag_multi_index_router import chain as rag_multi_index_router_chain
+
+add_routes(app, rag_multi_index_router_chain, path="/rag-multi-index-router")
+```
+
+(Optional) Let's now configure LangSmith. 
+LangSmith will help us trace, monitor and debug LangChain applications. 
+LangSmith is currently in private beta, you can sign up [here](https://smith.langchain.com/). 
+If you don't have access, you can skip this section
+
+
+```shell
+export LANGCHAIN_TRACING_V2=true
+export LANGCHAIN_API_KEY=<your-api-key>
+export LANGCHAIN_PROJECT=<your-project>  # if not specified, defaults to "default"
+```
+
+If you are inside this directory, then you can spin up a LangServe instance directly by:
+
+```shell
+langchain serve
+```
+
+This will start the FastAPI app with a server is running locally at 
+[http://localhost:8000](http://localhost:8000)
+
+We can see all templates at [http://127.0.0.1:8000/docs](http://127.0.0.1:8000/docs)
+We can access the playground at [http://127.0.0.1:8000/rag-multi-index-router/playground](http://127.0.0.1:8000/rag-multi-index-router/playground)  
+
+We can access the template from code with:
+
+```python
+from langserve.client import RemoteRunnable
+
+runnable = RemoteRunnable("http://localhost:8000/rag-multi-index-router")
+```