docs: Enhance GitHub Sign-In setup, customizations, and troubleshooting

[Zfinix]

Problem

The Sign in with GitHub docs lagged behind Google and Apple in structure and depth: no Prerequisites section, title-case headings inconsistent with sibling providers, no production deployment guidance, no troubleshooting page, stale screenshots, and a thin customizations page missing the server-side API hook, global auth user hooks, and parameter reference. The Web flow also predated the new FlutterWebAuth2CallbackRoute and relied on a manually-created auth.html even when Serverpod could serve the callback itself.

Solution

• Setup page -- Added Prerequisites, sentence-case headings, and a ## Publishing to production section with self-hosted and Serverpod Cloud (scloud password set) paths. Web flow refactored into Serverpod-hosted (using pod.configureFlutterWebAuth2CallbackRoute() and /auth/callback) and separately-hosted (static auth.html) sub-paths. Five new screenshots replace the stale set.
• Customizations page -- Renamed from 02-configuration.md to 02-customizations.md for parity with Google. Added GitHubIdpConfig vs GitHubIdpConfigFromPasswords intro, manual credential loading, server-side API access via getExtraGitHubInfoCallback, global onBeforeAuthUserCreated / onAfterAuthUserCreated hooks, and a parameter reference table.
• Troubleshooting page (new) -- Setup checklist and 11 scenarios covering redirect_uri_mismatch, callback handling for both web flows, email null, dev-vs-prod failures, installation scope, org OAuth restrictions, bad_verification_code, rate limits, permission re-authorization, and migration issues.

Requires serverpod_auth_idp_server 3.5.0-beta.8 or later for configureFlutterWebAuth2CallbackRoute. Depends on serverpod/serverpod#5133.