docs(feature-flags): add Dart client guide

[leoromanovsky]

Motivation

Customers using Feature Flags from Dart need a docs path that matches the released datadog_flags package. The client SDK landing page already covers Android, iOS, React Native, Unity, and browser SDKs, but it does not route Dart customers to an installation, initialization, evaluation, and telemetry guide.

Changes

This PR adds a Dart client guide under content/en/feature_flags/client/flutter.md. The filename and URL stay unchanged to keep the docs diff small, but the customer-facing label is Dart. The page documents the standalone datadog_flags package: installation, SDK initialization, client creation, evaluation context, typed flag evaluation, telemetry options, last-known assignment storage, shutdown, and a unit-testing pattern.

The page also includes an OpenFeature compatibility note: the current Dart package exposes an OpenFeature-compatible API, but it is not built on the OpenFeature Dart SDK. It directs customers to use the documented APIs directly while Datadog works on an OpenFeature-provider-based integration for Dart and Flutter.

The PR also adds Dart to the client SDK card grid, the Feature Flags side navigation, and the LaunchDarkly migration platform picker. The existing telemetry comparison table now lists the Dart option names for exposures and flag evaluation telemetry.

Decisions

This stack documents only the standalone Dart package. Flutter applications can use datadog_flags directly from Dart code, but the Flutter SDK plugin integration is intentionally left for a stacked documentation PR that follows DataDog/dd-sdk-flutter#1071.

The page stays in the existing plain Markdown Feature Flags client docs instead of adding Cdocs filters, matching the neighboring Android, iOS, React Native, Unity, and browser pages.

The OpenFeature wording is presented as a compatibility note rather than a limitation banner: it sets the correct implementation expectation without implying that customers should wait for the provider-based integration before using Feature Flags in Dart.

Validation

• git diff --check
• ruby -e 'require "yaml"; YAML.load_file("config/_default/menus/main.en.yaml"); ARGV.each { |f| YAML.load_file(f); puts "ok #{f}" }' content/en/feature_flags/client/_index.md content/en/feature_flags/client/flutter.md content/en/feature_flags/guide/migrate_from_launchdarkly.md
• python3 shortcode balance check for tabs, code blocks, and site-region blocks in touched Markdown files.
• Local Hugo preview build reached site rendering after a temporary GitHub HTTPS module-auth workaround, then failed on checkout-local generated Markdoc assets: partial "markdoc-assets.html" not found. This is not caused by the Dart page diff; make build-cdocs is unavailable in this checkout because Makefile.config and local/bin/js/cdocs-build.js are missing.

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/leo.romanovsky/docs-flutter-feature-flags/feature_flags/client/flutter

Modified Files

• https://docs-staging.datadoghq.com/leo.romanovsky/docs-flutter-feature-flags/feature_flags/client/
• https://docs-staging.datadoghq.com/leo.romanovsky/docs-flutter-feature-flags/feature_flags/guide/migrate_from_launchdarkly