Add link to customization docs (Azure#52686)

* Add link to customization docs

* delete docs

* fix script

Author: JoshLove-msft

eng/packages/http-client-csharp/docs/emitter.md renamed to eng/packages/http-client-csharp/.tspd/docs/emitter.md

@@ -0,0 +1,8 @@
+### Prerequisite
+
+- Install [Node.js](https://nodejs.org/download/) 20 or above. (Verify by running `node --version`)
+- Install [**.NET 8.0 SDK**](https://dotnet.microsoft.com/download/dotnet/8.0) for your specific platform. (or a higher version)
+
+### Customizing Generated Code
+
+For detailed instructions on how to customize the generated C# code, see the [Customization Guide](https://github.com/microsoft/typespec/blob/main/packages/http-client-csharp/.tspd/docs/customization.md).

eng/packages/http-client-csharp/docs/index.mdx renamed to eng/packages/http-client-csharp/.tspd/docs/index.mdx

@@ -8,6 +8,17 @@ TypeSpec library for emitting Azure libraries for C#.
 npm install @azure-typespec/http-client-csharp
 ```
 
+## Usage
+
+### Prerequisite
+
+- Install [Node.js](https://nodejs.org/download/) 20 or above. (Verify by running `node --version`)
+- Install [**.NET 8.0 SDK**](https://dotnet.microsoft.com/download/dotnet/8.0) for your specific platform. (or a higher version)
+
+### Customizing Generated Code
+
+For detailed instructions on how to customize the generated C# code, see the [Customization Guide](https://github.com/microsoft/typespec/blob/main/packages/http-client-csharp/.tspd/docs/customization.md).
+
 ## Emitter usage
 
 1. Via the command line

eng/packages/http-client-csharp/.tspd/docs/usage.md

@@ -31,7 +31,7 @@
     "lint:fix": "eslint . --fix",
     "prettier:fix": "prettier --write --config .prettierrc.json ./**/*.ts package.json",
     "prettier": "prettier --check --config .prettierrc.json ./**/*.ts package.json",
-    "regen-docs": "npm run build:emitter && tspd doc . --enable-experimental --skip-js"
+    "regen-docs": "npm run build:emitter && tspd doc . --output-dir .tspd/docs --enable-experimental --skip-js"
   },
   "files": [
     "dist/emitter/src/**",