Update docs with Sentry initialization callout (#14391)

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## DESCRIBE YOUR PR
This PR updates the Node.js and Bun JavaScript guides to make the `instrument.js` import timing more prominent. The previous introductory paragraph in the "Apply Instrumentation to Your App" (Node.js) and "Use" (Bun) sections has been replaced with a yellow warning callout.

This change ensures that users are clearly informed about the importance of initializing Sentry early to prevent issues with auto-instrumentation, database monitoring, agent monitoring, and tracing.

**Affected pages:**
- https://docs.sentry.io/platforms/javascript/guides/node/#apply-instrumentation-to-your-app
- https://docs.sentry.io/platforms/javascript/guides/bun/#use

**New text in callout:**
> Import `instrument.js` before any other modules to ensure Sentry initializes early. If you initialize later, auto-instrumentation and modules like database monitoring, agent monitoring, tracing may not work.

## IS YOUR CHANGE URGENT?  

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- [ ] Urgent deadline (GA date, etc.): <!-- ENTER DATE HERE -->
- [ ] Other deadline: <!-- ENTER DATE HERE -->
- [x] None: Not urgent, can wait up to 1 week+

## SLA

- Teamwork makes the dream work, so please add a reviewer to your PRs.
- Please give the docs team up to 1 week to review your PR unless you've added an urgent due date to it.
Thanks in advance for your help!

## PRE-MERGE CHECKLIST

*Make sure you've checked the following before merging your changes:*

- [ ] Checked Vercel preview for correctness, including links
- [ ] PR was reviewed and approved by any necessary SMEs (subject matter experts)
- [ ] PR was reviewed and approved by a member of the [Sentry docs team](https://github.com/orgs/getsentry/teams/docs)

## LEGAL BOILERPLATE

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Look, I get it

Co-authored-by: Cursor Agent <[email protected]>

Author: jaffrepaul

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

@@ -64,7 +64,11 @@ You can also manually capture performance data - see <PlatformLink to="/tracing/
 
 ## Use
 
-You need to import the `instrument.js` file before importing any other modules in your application. This is necessary to ensure that Sentry can automatically instrument all modules in your application:
+<Alert level='warning'>
+
+Import `instrument.js` before any other modules to ensure Sentry initializes early. If you initialize later, auto-instrumentation and modules like database monitoring, agent monitoring, tracing may not work.
+
+</Alert>
 
 ```javascript {filename: app.js}
 // Import this first!

platform-includes/getting-started-node/javascript.mdx

@@ -26,7 +26,11 @@ To import and initialize Sentry, create a file named `instrument.(js|mjs)` in th
 
 ### Apply Instrumentation to Your App
 
-Make sure to initialize Sentry before you require or import any other modules in your app. Otherwise, auto-instrumentation won't work for these modules.
+<Alert level='warning'>
+
+Import `instrument.js` before any other modules to ensure Sentry initializes early. If you initialize later, auto-instrumentation and modules like database monitoring, agent monitoring, tracing may not work.
+
+</Alert>
 
 <Alert level="info" title="Which module system are you using?">