docs: Rewrite Facebook sign-in setup with prerequisites and production steps

[Zfinix]

Problem

The Facebook sign-in setup page had gaps that stopped a developer from getting it working end to end:

• No prerequisites section, unlike the migrated Google, Apple, and GitHub setup pages.
• The web flow was missing the Login with the JavaScript SDK toggle. Without it, web sign-in fails at runtime with "JSSDK Option is Not Toggled".
• No Publishing to production section, so there was no guidance for going live (Live mode, production domains, release key hash, production credentials for self-hosted and Serverpod Cloud).
• The server-side steps were less explicit than the other providers (no file paths, no migration warning).

Solution

• Adds a Prerequisites section matching the other providers.
• Adds the Login with the JavaScript SDK toggle to the portal Settings and Web steps.
• Rewrites the server-side section to be explicit: file paths, FacebookIdpConfigFromPasswords() as the primary example, and a migration warning.
• Adds a Publishing to production section covering Live mode, production domains and release key hash, production credentials for self-hosted and Serverpod Cloud, and providing the App ID to the production Flutter build.
• Slims the duplicated custom-widget example in favor of a pointer to Customizing the UI.
• Refreshes the portal screenshots and adds captures for the create-app and JavaScript SDK steps.

Stacked on #667 (uses the renamed customizations page). Part of #503 and #507.