Remove redundant api-reference/README.md

This category index file was an orphaned remnant from PR #1813 that used
a different navigation approach with category landing pages.

During the IA redesign (PR #1845), similar category index files like
guides/advanced/README.md were deleted as "redundant" because docs/README.md
now serves as the single navigation hub.

The api-reference/README.md survived by accident and is now the only category
with such an index file, creating inconsistency.

Changes:
- Deleted docs/api-reference/README.md (7-line TOC)
- Updated docs/README.md reference to point directly to view-helpers-api.md
  (the main API reference file)
- Preserves existing folder-level link on line 50 that points to api-reference/

Rationale: Category folders don't need README indexes when the main docs hub
already provides navigation. This matches the pattern established when we
deleted guides/advanced/README.md.

Author: ihabadham

docs/README.md

@@ -21,7 +21,7 @@ Perfect if you're new to React on Rails
 Jump straight to what you need
 
 - **[Installation Guide](./getting-started/installation-into-an-existing-rails-app.md)** - Add to existing Rails app
-- **[API Reference](./api-reference/README.md)** - View helpers, configuration, JavaScript API
+- **[API Reference](./api-reference/view-helpers-api.md)** - View helpers, configuration, JavaScript API
 - **[Core Concepts](./core-concepts/how-react-on-rails-works.md)** - Architecture and SSR
 
 ### 🏗️ **Migrating from Other Solutions**