Native Windows is out of beta. Removes the early-beta warnings, headings,
and rough-edge framing across the README and docs (EN + zh-Hans), keeping
the WSL2-only dashboard PTY caveat. Historical RELEASE_v0.14.0.md notes are
left intact since they accurately describe the state at that release.
- README: Windows install + cross-platform notes
- index.mdx, installation.md: headings, warning admonitions, parity note
- windows-native.md: title/sidebar_label/warning, provider-hunting tip
- contributing.md, nous-portal.md: cross-platform / Portal parity prose
- Repoint cross-links to the renamed installation#windows-native-powershell
anchor (EN) and #windows原生powershell (zh, also fixes pre-existing drift)
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