(feat) Allow using include to include external YAML files in a config.yaml (#6922)

* add helper to process inlcudes directive on yaml

* add doc on config management

* unit tests for `include` on config.yaml

docs/my-website/docs/proxy/config_management.md

@@ -0,0 +1,59 @@
+# File Management
+
+## `include` external YAML files in a config.yaml 
+
+You can use `include` to include external YAML files in a config.yaml. 
+
+**Quick Start Usage:**
+
+To include a config file, use `include` with either a single file or a list of files. 
+
+Contents of `parent_config.yaml`:
+```yaml
+include:
+  - model_config.yaml # 👈 Key change, will include the contents of model_config.yaml
+
+litellm_settings:
+  callbacks: ["prometheus"] 
+```
+
+
+Contents of `model_config.yaml`:
+```yaml
+model_list:
+  - model_name: gpt-4o
+    litellm_params:
+      model: openai/gpt-4o
+      api_base: https://exampleopenaiendpoint-production.up.railway.app/
+  - model_name: fake-anthropic-endpoint
+    litellm_params:
+      model: anthropic/fake
+      api_base: https://exampleanthropicendpoint-production.up.railway.app/
+
+```
+
+Start proxy server 
+
+This will start the proxy server with config `parent_config.yaml`. Since the `include` directive is used, the server will also include the contents of `model_config.yaml`.
+```
+litellm --config parent_config.yaml --detailed_debug
+```
+
+
+
+
+
+## Examples using `include`
+
+Include a single file:
+```yaml
+include:
+  - model_config.yaml
+```
+
+Include multiple files:
+```yaml
+include:
+  - model_config.yaml
+  - another_config.yaml
+```

docs/my-website/docs/proxy/configs.md

@@ -2,7 +2,7 @@ import Image from '@theme/IdealImage';
 import Tabs from '@theme/Tabs';
 import TabItem from '@theme/TabItem';
 
-# Proxy Config.yaml
+# Overview
 Set model list, `api_base`, `api_key`, `temperature` & proxy server settings (`master-key`) on the config.yaml. 
 
 | Param Name           | Description                                                   |

docs/my-website/sidebars.js

@@ -32,7 +32,7 @@ const sidebars = {
         {
           "type": "category", 
           "label": "Config.yaml",
-          "items": ["proxy/configs", "proxy/config_settings"]
+          "items": ["proxy/configs", "proxy/config_management", "proxy/config_settings"]
         },
         {
           type: "category",