docs: Reshape the Authentication provider pages for 4.0

[developerjamiu]

Follow-up to #728 and #729. This PR normalizes the shape of the Authentication provider pages so every provider group follows the same structure. Content changes are limited to what the restructure requires plus fixes found in the pre-PR review.

Structure

• One customizations page per OAuth group. For anonymous, Google, Apple, Facebook, GitHub, and Microsoft, the old configuration/customizations page and the separate "Customizing the UI" page are merged into a single customizations page (server configuration, then app configuration, then UI). Email and Firebase keep their separate UI pages since their scope differs.
• Microsoft brought up to group parity. Its configuration page is renamed to customizations, and it gains a troubleshooting page matching its siblings (16 entries, each verified against the framework source).
• Troubleshooting pages renumbered to 03 across the merged groups, now that the UI pages are gone.
• Passkey UI stub removed. Its content already lives on the setup page.
• Custom providers overview rewritten from a stub into a real entry page (what a custom provider consists of, OAuth2 path vs everything else).
• auth.html has a single home on the core setup page; other mentions now point there.

Content fixes from the review pass

• Google web setup used the /app serving path throughout; default projects (no website option) serve the built app at /. Corrected in four places, with the website-option case called out.
• The google-services.json fallback on Android only works with the Firebase Gradle plugin; plain projects must pass client IDs in code or with --dart-define.
• The anonymous rate-limit snippet now shows the import that RateLimit actually requires.
• Facebook gained the credential-loading section its setup page promised, plus a FacebookIdpConfig parameter reference matching the Google/GitHub tables.
• The Apple email/name persistence story is now consistent between setup and troubleshooting (Serverpod stores the values automatically on first sign-in and does not backfill them later).
• Smaller flow and wording fixes across the group: consistent Related blocks on all five troubleshooting pages, accurate headings for the missing-password startup crash, and cross-links between the split scopes/server-API sections.

Redirects

The removed URLs (customizing-the-ui pages, microsoft/configuration, anonymous/configuration) only exist in the /next version, so @docusaurus/plugin-client-redirects rejects redirect entries for them while 3.4.0 is the served default. The full redirect list is recorded and will land at the 4.0 GA cutover.