feat: Apple Calendar settings UX improvements and user-facing docs

[diogobaltazar]

Summary

Three related improvements to the Apple Calendar integration:

1. Settings icon — Replace the current sun/radial-lines icon in App.jsx with a conventional gear/cog icon so users can find settings intuitively.

2. Settings panel copy — Replace the existing helper paragraph ("For iCloud, use an app-specific password from your Apple account security settings.") with: "Generate your password from account.apple.com" followed by a small link icon pointing to https://support.apple.com/en-us/102654 (opens in a new tab).

3. User-facing docs — Add a new apple-calendar.mdx page to fern/pages/ registered under the existing "Overview" section in fern/docs.yml, covering: what the integration does, how to generate an app-specific password (link to Apple's support page), how to connect via the settings panel, how to select a writable calendar, and how to disconnect.

Acceptance criteria

• Settings icon is a recognisable cog/gear
• Helper text reads "Generate your password from account.apple.com" with a link icon to https://support.apple.com/en-us/102654 opening in a new tab
• fern/pages/apple-calendar.mdx exists and covers the full connection flow
• Page is registered in fern/docs.yml under the "Overview" section
• No regressions to existing Apple Calendar connect/disconnect flow