Remove confusing "(Free)" label from Turbo Basic Setup heading

The "(Free)" label in "### Basic Setup (Free)" was confusing because:
- Appears before any mention of Pro features
- Not used elsewhere in the codebase
- Readers don't know what it's contrasting with yet
- The Pro callout box later provides sufficient contrast

Changed to simply "### Basic Setup" to match patterns used in other docs
like streaming-server-rendering.md.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ihabadham

docs/building-features/turbolinks.md

@@ -10,7 +10,7 @@
 
 Turbo is the modern replacement for Turbolinks, providing fast navigation through your Rails app without full page reloads.
 
-### Basic Setup (Free)
+### Basic Setup
 
 **1. Install Turbo**