Update welcome page title and subtitle per PR feedback

- Change title from 'Welcome to the Fern docs starter' to 'Welcome to Fern!'
- Change subtitle to action-oriented tagline: 'Everything you need to build the best developer experience'

Co-Authored-By: Sarah Bawabe <sarah@buildwithfern.com>

Author: devin-ai-integration[bot]

fern/docs/pages/welcome.mdx

@@ -1,6 +1,6 @@
 ---
-title: Welcome to the Fern docs starter
-subtitle: A template for building beautiful API documentation with Fern
+title: Welcome to Fern!
+subtitle: Everything you need to build the best developer experience
 slug: welcome
 ---