feat: rename Changelog to What's New (UX-93)

[betzlermeow]

Description

• Renamed tab display name from "Changelog" to "What's New" in fern/docs.yml
• Updated tab slug from changelog to whats-new in fern/docs.yml
• Updated existing redirect for /documentation/general/changelog to point to /whats-new in fern/docs.yml
• Added new redirect from /changelog to /whats-new in fern/docs.yml
• Updated frontmatter slug from changelog to whats-new in fern/changelog/overview.mdx
• Updated subscription text from "Get the (almost) daily changelog" to "Subscribe to the latest product updates" in fern/changelog/overview.mdx
• Updated Customer.io form success URL to https://docs.vapi.ai/whats-new in fern/changelog/overview.mdx

Testing Steps

• Run the app locally using fern docs dev or navigate to preview deployment
• Ensure that the changed pages and code snippets work
• Verify /whats-new URL loads the renamed page
• Verify /changelog redirects to /whats-new
• Verify subscription form success URL points to /whats-new