chore: better README on the Astro package

[Nsttt]

Summary

• add a beta callout that asks users to file feedback and bugs via GitHub Issues
• rewrite the top of the README to lead with @module-federation/astro instead of repo internals
• surface the example apps earlier and move repo structure details lower in the document

Why

The README opened with repository implementation details instead of the package value proposition. This makes the project harder to scan for users evaluating whether @module-federation/astro solves their problem.

Impact

Users landing on the repo now see what the package is, what it supports, how to use it, and where to try it before they see workspace-specific implementation details.

Validation

• reviewed rendered Markdown structure locally
• no code or runtime changes
• no tests run

[Copilot]

Pull request overview

This PR updates the repository root README to prioritize the public @module-federation/astro package narrative (value proposition, beta status, and quick paths to try examples) instead of leading with repo/workspace implementation details.

Changes:

• Rewrites the README introduction to lead with @module-federation/astro and adds a beta “IMPORTANT” callout directing users to GitHub Issues.
• Adds an earlier “Example apps” section with host/remote ports and the pnpm dev entrypoint.
• Moves repository-structure details into a dedicated “Repo layout” section near the bottom.

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