feat(js): Add propagateTraceparent option documentation (#14805)

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## DESCRIBE YOUR PR

To be merged after
getsentry/sentry-javascript#17509 was released

This PR adds documentation for the new, browser-SDK-only,
`propagateTraceparent` option (see [develop
docs](https://develop.sentry.dev/sdk/telemetry/traces/#propagatetraceparent)).
It also adjusts our "Dealing with CORS Issues" page to include
`traceparent` in the `CORS` configuration if users opted into the
option.

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## EXTRA RESOURCES

- [Sentry Docs contributor guide](https://docs.sentry.io/contributing/)

Author: Lms24

docs/platforms/javascript/common/configuration/options.mdx

@@ -443,6 +443,27 @@ By default, no spans are ignored.
 
 </SdkOption>
 
+<PlatformCategorySection supported={['browser']}>
+
+<SdkOption name="propagateTraceparent" type='boolean' defaultValue='false'>
+
+_Available since: `10.10.0`_
+
+If set to `true`, the SDK adds the [W3C `traceparent` header](https://www.w3.org/TR/trace-context/) to outgoing Http requests made via `fetch` or `XMLHttpRequest`.
+This header is attached in addition to the `sentry-trace` and `baggage` headers.
+Set this option to `true` if your backend services are instrumented with e.g. OpenTelemetry or other W3C Trace Context compatible libraries and you want to continue traces from the client.
+
+**Important:** Make sure that your backend services' CORS configuration allows the `traceparent` header.
+Otherwise, requests might be blocked.
+Use the [`tracePropagationTargets` option](#tracepropagationtargets) to control which requests the `traceparent` header is attached to.
+See <PlatformLink to="/tracing/distributed-tracing/dealing-with-cors-issues/">Dealing with CORS Issues</PlatformLink> for more information.
+
+Note that this option is only available for browser SDKs.
+
+</SdkOption>
+
+</PlatformCategorySection>
+
 ## Session Replay Options
 
 <SdkOption name="replaysSessionSampleRate" type='number' categorySupported={['browser']}>

docs/platforms/javascript/common/tracing/distributed-tracing/dealing-with-cors-issues/index.mdx

@@ -20,7 +20,7 @@ If your frontend and backend are hosted on different domains (for example, your
 
 ## Understanding Trace Propagation
 
-Sentry doesn't attach the `sentry-trace` and `baggage` headers to every outgoing request. Instead, it only attaches these headers to requests whose URLs match the patterns specified in the `tracePropagationTargets` configuration option.
+Sentry doesn't attach the `sentry-trace` and `baggage` (and optionally `traceparent`) headers to every outgoing request. Instead, it only attaches these headers to requests whose URLs match the patterns specified in the `tracePropagationTargets` configuration option.
 
 By default, `tracePropagationTargets` is set to `['localhost', /^\//]`, which means trace headers are only attached to:
 - Requests containing `localhost` in their URL (e.g., `http://localhost:3000/api`)
@@ -55,6 +55,14 @@ Access-Control-Allow-Headers: sentry-trace, baggage
 
 Your server's exact configuration will depend on your setup, but the important part is allowing both the `sentry-trace` and `baggage` headers.
 
+### W3C traceparent header
+
+If you set <PlatformLink to="/configuration/options#propagateTraceparent">`propagateTraceparent` option</PlatformLink> to `true`, you also need to allow the `traceparent` header:
+
+```
+Access-Control-Allow-Headers: sentry-trace, baggage, traceparent
+```
+
 ## Disabling Trace Propagation
 
 If you want to disable distributed tracing entirely and ensure no trace headers are sent: