docs: Add macOS Keychain Sharing guide for authentication (#4512)

[abdulawalarif]

Summary

• Adds a new guide, Set up authentication on macOS, documenting the Keychain Sharing entitlement required for sign-in sessions to persist on macOS.
• Covers the default fix (empty keychain-access-groups in entitlements files), verification steps, Xcode setup, and a local-development alternative using the login keychain.
• Adds troubleshooting for the PlatformException(-34018, A required entitlement isn't present, ...) error reported in Email auth does not work out of the box on MacOS and error message is unclear. serverpod#4512.
• Links to the new page from Get started, Setup, Authentication basics, and the Facebook provider macOS section.

Closes serverpod/serverpod#4512 (docs portion).

Test plan

• Run npm run build locally and confirm the site builds without broken links
• Confirm the new page appears under Authentication in the sidebar
• Verify cross-links resolve: ./macos-authentication from get-started, setup, basics, and Facebook setup
• Review the entitlements XML snippet and troubleshooting steps against a macOS Flutter project