docs(js): Update Astro quick start guide

[inventarSarah]

DESCRIBE YOUR PR

This branch contains the updated Astro quick start guide.

Note:

• I will move content under "Server Instrumentation" to other files in a separate PR to keep PR sizes small
• Once this is all done, I will make the updates to the Astro for Cloudflare guide

Closes: #15832

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Project	Deployment	Review	Updated (UTC)
sentry-docs	Ready	Preview, Comment	Jan 20, 2026 6:22am

1 Skipped Deployment

Project	Deployment	Review	Updated (UTC)
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[s1gr1d]

Suggested change

	### Customize Server Instrumentation for Manual Middleware (Optional)
	### Customize Server Instrumentation with Manual Middleware (Optional)

I would rather say it's customizing it "with" the middleware

[coolguyzone]

LGTM!