feat(flags): document custom featureFlagsIntegration and refactor all JS integration docs

docs/platforms/javascript/common/configuration/integrations/featureflags.mdx

@@ -0,0 +1,71 @@
+---
+title: FeatureFlags
+description: "Learn how to attach custom feature flag data to Sentry error events."
+notSupported:
+  - javascript.aws-lambda
+  - javascript.azure-functions
+  - javascript.bun
+  - javascript.capacitor
+  - javascript.cloudflare
+  - javascript.connect
+  - javascript.cordova
+  - javascript.deno
+  - javascript.electron
+  - javascript.express
+  - javascript.fastify
+  - javascript.gcp-functions
+  - javascript.hapi
+  - javascript.koa
+  - javascript.nestjs
+  - javascript.nodejs
+  - javascript.wasm
+---
+
+<PlatformContent includePath="feature-flags/prerelease-alert" />
+
+<Alert level="info">
+
+This integration only works inside a browser environment. If you are using a specific
+feature flag provider, refer to the supported list [here](/platforms/javascript/feature-flags/#enable-evaluation-tracking) before setting up this integration.
+
+</Alert>
+
+The Feature Flags integration allows you to manually track feature flag evaluations through an API. These evaluations are held in memory and sent to Sentry when an error occurs.
+**At the moment, we only support boolean flag evaluations.**
+
+_Import names: `Sentry.featureFlagsIntegration` and `type Sentry.FeatureFlagsIntegration`_
+
+## Install
+
+Install your platform's Sentry SDK from npm.
+
+## Configure
+
+```JavaScript
+import * as Sentry from '@sentry/browser';
+
+Sentry.init({
+  dsn: "___PUBLIC_DSN___",
+  integrations: [Sentry.featureFlagsIntegration()]
+});
+```
+
+## Verify
+
+The integration is tested by calling the `addFeatureFlag` method before capturing an exception.
+
+```JavaScript
+import * as Sentry from '@sentry/browser';
+
+const flagsIntegration = Sentry.getClient()?.getIntegrationByName<Sentry.FeatureFlagsIntegration>('FeatureFlags');
+if (flagsIntegration) {
+  flagsIntegration.addFeatureFlag('hello', false);
+} else {
+  // check your configure step
+}
+Sentry.captureException(new Error('broke'));
+```
+
+Visit the Sentry website and confirm that your error event has recorded the feature flag "hello" and its value "false".
+
+<PlatformContent includePath="feature-flags/next-steps" />

docs/platforms/javascript/common/configuration/integrations/launchdarkly.mdx

@@ -69,7 +69,7 @@ import * as LaunchDarkly from 'launchdarkly-js-client-sdk';
 // You may have to wait for your client to initialize before doing this.
 ldClient?.variation("hello", false);
 
-Sentry.captureException(Exception("Something went wrong!"))
+Sentry.captureException(new Error("Something went wrong!"))
 ```
 
 Visit the Sentry website and confirm that your error event has recorded the feature flag "hello" and its value "false".

docs/platforms/javascript/common/configuration/integrations/openfeature.mdx

@@ -73,7 +73,7 @@ import { OpenFeature } from '@openfeature/web-sdk';
 const client = OpenFeature.getClient();
 const result = client.getBooleanValue('hello', false);
 
-Sentry.captureException(Exception("Something went wrong!"))
+Sentry.captureException(new Error("Something went wrong!"))
 ```
 
 Visit the Sentry website and confirm that your error event has recorded the feature flag "hello" and its value "false".

docs/platforms/javascript/common/feature-flags/index.mdx

@@ -34,4 +34,6 @@ Evaluation tracking requires enabling an SDK integration. Integrations are provi
 - [OpenFeature](/platforms/javascript/configuration/integrations/openfeature/)
 - [LaunchDarkly](/platforms/javascript/configuration/integrations/launchdarkly/)
 
+To manually track evaluations through an API, see the custom [FeatureFlags](/platforms/javascript/configuration/integrations/featureflags/) integration.
+
 <PlatformContent includePath="feature-flags/enable-change-tracking" />