docs: document the Branding module across README, docs site, and website

[antosubash]

Summary

The Branding module shipped in #273 without any documentation outside its screenshots README (docs/branding/README.md). This PR syncs the docs with it:

• New docs page docs/site/guide/branding.md — covers the /branding/manage admin page and Branding.Manage permission, how values are stored via ISettingsContracts under branding.* keys (intentionally not SettingDefinitions), server-side head injection, the branding Inertia shared prop, the API endpoints (including the anonymous asset-serve path and why it exists), and the security hardening (no SVG, nosniff, javascript: link sanitization, 2 MB upload cap). Registered in the VitePress sidebar under Infrastructure.
• Inertia guide — documents the IInertiaHeadContributor extension point introduced by the Branding PR, adds the HEAD_CONTRIBUTIONS comment placeholder to the HTML-shell walkthrough (the snippet had gone stale vs. the real wwwroot/index.html), and mentions the substitution in the request-time rendering description.
• README — adds Branding to the module list in the project-structure section.
• Project Structure page — adds Branding/ to the modules/ tree, plus Identity/, Keycloak/, and Notifications/, which were already missing from that tree.
• Marketing site (website/index.html) — adds a Branding card to the "Batteries included" module showcase and bumps the built-in module count from 16 to 17.

Verification

• npm run build in docs/site (VitePress) passes, including its dead-link check for the new page and sidebar entry.