docs: update package manager command from beta to rc

[chenjiahan]

Summary

Update package manager command from npm create rspress@beta to npm create rspress@rc.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Copilot]

Pull request overview

This PR updates the package manager command in the documentation from the beta version to the release candidate (rc) version of rspress.

• Updated the PackageManagerTabs command from create rspress@beta to create rspress@rc

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[github-actions]

Rsdoctor Bundle Diff Analysis

📁 web

Path: website/doc_build/web/rsdoctor-data.json

📌 Baseline Commit: e76258a3ea | PR: #1802

Metric	Current	Baseline	Change
📊 Total Size	14.4 MB	14.4 MB	-2.0 B (-0.0%)
📄 JavaScript	13.8 MB	13.8 MB	-2.0 B (-0.0%)
🎨 CSS	109.9 KB	109.9 KB	0 B (0.0%)
🌐 HTML	0 B	0 B	N/A
📁 Other Assets	496.9 KB	496.9 KB	0 B (0.0%)

📦 Download Diff Report: web Bundle Diff

Generated by Rsdoctor GitHub Action