Add docs: observability, cache, adapters, and APIs
Add and update framework documentation to introduce runtime features and integrations. New pages: AdapterRegistry, Cache, CloudflareTurnstile, CrudIncludePreloader, CrudResourceMetadata, EmptyResponse, FilesystemMetadata, HttpCache, MeilisearchClient, Observability, ObservabilityEvent*, OpenAIClient, OpenTelemetryHttpExporter, ResendMailer, UiRenderer, and others. Update API, Application, Env, Configuration-file, CrudController, CrudResourceConfig, ApiExposable, Database, Form, FormControl, ErrorCodes and related docs to reference these additions: adapter registry usage, process-local metadata/cache (FilesystemMetadata, CrudResourceMetadata, Env caching), include preloader for bulk relation loading, explicit HttpCache/EmptyResponse examples, observability spans and debug headers, and expanded .env variables for cache, observability, exporters, Stripe/OpenAI/Resend/Meilisearch/Cloudflare Turnstile. Also document best practices for clearing process-local caches in long-running processes and note that DB/cache helpers emit spans when observability is enabled.