Document React appEntrypoint prop

[meyer]

Description (required)

This PR contains React integration docs updates for this PR: withastro/astro#15368

I documented the new appEntrypoint prop and added specific details for accessing Astro.locals with the locals prop.

For Astro version: 6.0.0. See astro PR 15368.

[netlify]

✅ Deploy Preview for astro-docs-2 ready!

Built without sensitive environment variables

Name	Link
🔨 Latest commit	41f4db9
🔍 Latest deploy log	https://app.netlify.com/projects/astro-docs-2/deploys/69a35e812f26da0007fd7239
😎 Deploy Preview	https://deploy-preview-13208--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 project 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en/guides/integrations-guide/react.mdx	Source changed, localizations will be marked as outdated.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]

I'm so sorry, but it looks like this is not making it in pre-v6, and our docs repo is going to update majorly after v6 so it will be safer to re-draft this if/when the feature is closer to being released. Closing for now, and the content can be added in a separate PR later when the feature is approved!