docs(js): Review and update Fastify quick start guide (#14249)

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## DESCRIBE YOUR PR
Updated the Fastify quick start guide to be consistent with our other
quick start guides.

~I want to merge #14218 first, as this branch contains changes that are
also needed for the changes in this pr~

Closes #14246

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## PRE-MERGE CHECKLIST

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- [ ] Checked Vercel preview for correctness, including links
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## EXTRA RESOURCES

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

Author: inventarSarah

docs/platforms/javascript/guides/fastify/index.mdx

@@ -1,6 +1,6 @@
 ---
 title: Fastify
-description: "This guide explains how to set up Sentry in your Fastify application."
+description: "Learn how to manually set up Sentry in your Fastify app and capture your first errors."
 sdk: sentry.javascript.node
 fallbackGuide: javascript.node
 categories:

platform-includes/getting-started-verify/javascript.fastify.mdx

@@ -1,5 +1,32 @@
+### Issues
+
+First, let's make sure Sentry is correctly capturing errors and creating issues in your project. Add the following code snippet to your main application file; it defines a route that will deliberately trigger an error when called:
+
 ```javascript
 app.get("/debug-sentry", function mainHandler(req, res) {
   throw new Error("My first Sentry error!");
 });
 ```
+
+<OnboardingOption optionId="performance">
+
+### Tracing
+
+To test your tracing configuration, update the previous code snippet by starting a trace to measure the time it takes for the execution of your code:
+
+```javascript
+app.get("/debug-sentry", async (request, reply) => {
+  await Sentry.startSpan(
+    {
+      op: "test",
+      name: "My First Test Transaction",
+    },
+    async () => {
+      await new Promise((resolve) => setTimeout(resolve, 100)); // Wait for 100ms
+      throw new Error("My first Sentry error!");
+    }
+  );
+});
+```
+
+</OnboardingOption>