docs: add supported headers reference documentation (#1488)

s## Add comprehensive HTTP headers documentation

This PR adds a new documentation page that provides a complete reference of all HTTP headers supported by Bifrost gateway. The documentation includes context keys, types, descriptions, and usage examples for each header.

## Changes

- Added new documentation page `docs/providers/supported-headers.mdx` with comprehensive header reference
- Updated navigation in `docs/docs.json` to include the new page
- Enhanced existing documentation in provider configuration and context keys pages to reference the new headers documentation
- Added documentation for the passthrough extra parameters feature (`x-bf-passthrough-extra-params` header)

## Type of change

- [ ] Bug fix
- [ ] Feature
- [ ] Refactor
- [x] Documentation
- [ ] Chore/CI

## Affected areas

- [ ] Core (Go)
- [ ] Transports (HTTP)
- [ ] Providers/Integrations
- [ ] Plugins
- [ ] UI (Next.js)
- [x] Docs

## How to test

The documentation can be tested by running the docs site locally:

```sh
cd docs
npm install
npm run dev
```

Then navigate to the new page at `/providers/supported-headers` to verify the content.

## Breaking changes

- [x] No

## Related issues

Addresses the need for comprehensive header documentation to help users understand all available request configuration options.

## Checklist

- [x] I read `docs/contributing/README.md` and followed the guidelines
- [x] I updated documentation where needed
- [x] I verified builds succeed (Go and UI)

Author: akshaydeo

core/schemas/chatcompletions.go

@@ -21,6 +21,13 @@ func (cr *BifrostChatRequest) GetRawRequestBody() []byte {
 	return cr.RawRequestBody
 }
 
+func (cr *BifrostChatRequest) GetExtraParams() map[string]interface{} {
+	if cr.Params == nil {
+		return make(map[string]interface{}, 0)
+	}
+	return cr.Params.ExtraParams
+}
+
 // BifrostChatResponse represents the complete result from a chat completion request.
 type BifrostChatResponse struct {
 	ID                string                     `json:"id"`

docs/docs.json

@@ -114,7 +114,8 @@
               "providers/provider-routing",
               "providers/reasoning",
               "providers/performance",
-              "providers/custom-providers"
+              "providers/custom-providers",
+              "providers/request-options"
             ]
           },
           {
@@ -492,6 +493,10 @@
     ]
   },
   "redirects": [
+    {
+      "source": "/providers/supported-headers",
+      "destination": "/providers/request-options"
+    },
     {
       "source": "/features/unified-interface",
       "destination": "/providers/supported-providers/overview"