Update server-islands.mdx to add caching section

[louisescher]

Description (required)

This PR includes a section about Caching Server Islands, as discussed on Talking and Doc'ing.

Related issues & labels (optional)

• Closes Document URL length-based request method switching for Server Islands v5 #10125
• Suggested label: add new content

[netlify]

✅ Deploy Preview for astro-docs-2 ready!

Built without sensitive environment variables

Name	Link
🔨 Latest commit	856e725
🔍 Latest deploy log	https://app.netlify.com/sites/astro-docs-2/deploys/6765a2b698b5760008789318
😎 Deploy Preview	https://deploy-preview-10466--astro-docs-2.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[astrobot-houston]

Lunaria Status Overview

🌕 This pull request will trigger status changes.

Learn more

By default, every PR changing files present in the Lunaria configuration's files property will be considered and trigger status changes accordingly.

You can change this by adding one of the keywords present in the ignoreKeywords property in your Lunaria configuration file in the PR's title (ignoring all files) or by including a tracker directive in the merged commit's description.

Tracked Files

File	Note
en/guides/server-islands.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[sarah11918]

Thanks Louis! Great addition to the docs! 💪