[Agents] Add authentication section to Agents docs

Adds a dedicated Authentication section with focused pages for each approach:

- Overview (authentication/index.mdx) with decision table and best practices.
- Token and middleware auth (authentication/token-and-middleware.mdx) covering
  static token validation, JWT verification, passing tokens from clients,
  per-instance authorization, and Hono middleware.
- Workers OAuth Provider (authentication/oauth-provider.mdx) covering OAuth 2.1
  authorization server setup, third-party IdP integration (GitHub example),
  and token scoping and revocation.
- Full-stack auth with Better Auth (authentication/better-auth.mdx) with a full
  tutorial for user management, D1 setup, JWT issuance, and client integration.

This replaces the stale PR #28654. Incorporates review feedback:

- Removed links to the cloudflare/agents auth-agent example from
  better-auth.mdx; the example was rewritten as a lightweight JWT demo and no
  longer demonstrates Better Auth.
- Corrected the 'Lazy initialization' note in better-auth.mdx. Environment
  bindings are available at module scope via 'import { env } from
  "cloudflare:workers"'. The real reason for the lazy singleton is that
  betterAuth()'s init() does async I/O (telemetry filesystem detection via
  dynamic import("fs/promises")) that only resolves inside a request context.
- Added a 'Complete example' link to token-and-middleware.mdx pointing at the
  auth-agent example, since that page describes the JWT pattern the example
  now demonstrates.
- Updated the auth-agent description in index.mdx to reflect that it
  demonstrates JWT auth, not Better Auth.

Author: Muhammad Ali