docs: add beta installation instructions to README

[developerkunal]

🔧 Changes

Adds a collapsible "Installing the Beta" section to the README so users can install and try pre-release builds without cluttering the primary (stable) install flow. The section is nested at the end of Installation and mirrors the beta branch conventions:

• Beta warning callouts (not yet GA; device-authorization login not fully supported, recommend M2M client credentials).
• Homebrew: brew install auth0-beta with a note that it installs as the auth0 binary.
• cURL: uses the /beta/install.sh script URL.
• Scoop: scoop install auth0-beta with the same binary note.
• Powershell: fetches the latest pre-release via prerelease -eq $true.
• Go: shows both @beta (tip of the beta branch) and a pinned @v1.33.0-beta.0 release, plus a note that @latest skips pre-releases.
• Manual: points to the releases page pre-release and reuses the existing Manual steps.

📚 References

N/A

🔬 Testing

Documentation-only change. Rendered the Markdown to confirm the <details> block collapses correctly and all install snippets display as expected.

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)