chore(docs): relicense Fern stylesheet under Apache 2.0

[drew]

Summary

Relicense the Fern custom stylesheet under Apache-2.0 so it can be included in the OpenShell open-source release.

Related Issue

No issue required: this is a localized license-header correction requested during OSRB review (NvBug 5544518).

Changes

• Replace LicenseRef-NvidiaProprietary with Apache-2.0 in fern/main.css
• Remove the proprietary-use restriction from the file header
• Retain the existing NVIDIA copyright attribution

Testing

• mise run pre-commit passes
• Unit tests added/updated (not applicable: header-only change)
• E2E tests added/updated (not applicable: header-only change)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

[github-actions]

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