doc to describe the step-by-step for low cost deployment

docs/deploy_lowcost.md

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+# Deploying with minimal costs
+
+This AI RAG chat application is designed to be easily deployed using the Azure Developer CLI, which provisions the infrastructure according to the Bicep files in the `infra` folder. Those files describe each of the Azure resources needed, and configures their SKU (pricing tier) and other parameters. Many Azure services offer a free tier, but the infrastructure files in this project do *not* default to the free tier as there are often limitations in that tier.
+
+However, if your goal is to minimize costs while prototyping your application, follow the steps below *before* running `azd up`. Once you've gone through these steps, return to the [deployment steps](../README.md#deployment).
+
+[📺 Live stream: Deploying from a free account]()
+
+1. Log in to your Azure account using the Azure Developer CLI:
+
+    ```shell
+    azd auth login
+    ```
+
+1. Create a new azd environment for the free resource group:
+
+    ```shell
+    azd env new
+    ```
+
+    Enter a name that will be used for the resource group.
+    This will create a new folder in the `.azure` folder, and set it as the active environment for any calls to `azd` going forward.
+
+1. Use the free tier of **Azure AI Document Intelligence** (aka [Form Recognizer](https://learn.microsoft.com/en-us/azure/ai-services/document-intelligence/overview?view=doc-intel-4.0.0)):
+
+    ```shell
+    azd env set AZURE_FORMRECOGNIZER_SERVICE_SKU F0
+    ```
+
+1. Use the free tier of **Azure AI Search**:
+
+    ```shell
+    azd env set AZURE_SEARCH_SERVICE_SKU free
+    azd env set AZURE_SEARCH_SEMANTIC_RANKER disabled
+    ```
+
+    Limitations:
+    1. You are only allowed one free search service across all regions.
+    If you have one already, either delete that service or follow instructions to
+    reuse your [existing search service](../README.md#use-existing-resources).
+    2. The free tier does not support semantic ranker. Note that will generally result in [decreased search relevance](https://techcommunity.microsoft.com/t5/ai-azure-ai-services-blog/azure-ai-search-outperforming-vector-search-with-hybrid/ba-p/3929167).
+
+1. Turn off **Azure Monitor** (Application Insights):
+
+    ```shell
+    azd env set AZURE_USE_APPLICATION_INSIGHTS false
+    ```
+
+    Application Insights is quite inexpensive already, so turning this off may not be worth the costs saved, but it is an option for those who want to minimize costs.
+
+1. (Optional) Use **OpenAI.com** instead of Azure OpenAI.
+
+    You can create a free account in OpenAI and [request a key to use OpenAI models](https://platform.openai.com/docs/quickstart/create-and-export-an-api-key). Once you have this, you can disable the use of Azure OpenAI Services, and use OpenAI APIs.
+
+    ```shell
+    azd env set USE_AOAI false
+    azd env set USE_VISION false
+    azd env set OPENAI_CHATGPT_DEPLOYMENT gpt-4o-mini
+    azd env set OPENAI_API_KEY <your openai.com key goes here>    
+    ```
+
+    ***Note:** Both Azure OpenAI and openai.com OpenAI accounts will incur costs, based on tokens used, but the costs are fairly low for the amount of sample data (less than $10).*
+
+1. Once you've made the desired customizations, follow the steps in the README [to run `azd up`](../README.md#deploying-from-scratch). We recommend using "eastus" as the region, for availability reasons.