docs(fern): migrate OpenShell docs to Fern

[lbliii]

Summary

This PR migrates the OpenShell docs to Fern and carries the follow-up cleanup needed to make the migrated site usable and close to the existing docs experience. It focuses on navigation, content structure, code block behavior, and landing-page polish, while leaving CI workflow work out of this PR.

Related Issue

• Related to OS-40

Changes

• Update navigation structure, labels, slugs, redirects, and section/page titles for the migrated Fern docs
• Convert tutorial walkthroughs to Fern Steps and fix malformed Tabs blocks
• Normalize code fences from console and bash to shell, and remove copied shell prompt prefixes so pasted commands run cleanly
• Improve the landing page with tighter badge spacing, a fixed inline animated terminal, llms-only / llms-ignore fallback content, and a global alpha announcement banner
• Clean up Fern page content and titles across tutorials, sandboxes, inference, security, and reference pages to better match the intended docs experience after migration
• Revert source docs/ migration edits so this PR stays Fern-only
• Exclude GitHub workflow / CI preview changes from this PR; those are kept separate on os-40-doc-publishing

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)
• fern check
• Manual review in local Fern preview for navigation, tutorial steps, tabs, landing page, and banner styling

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

[github-actions]

All contributors have signed the DCO ✍️ ✅
_{Posted by the DCO Assistant Lite bot.}

[lbliii]

I have read the DCO document and I hereby sign the DCO.

[pimlock]

Preview https://nvidia-preview-pr-780.docs.buildwithfern.com/openshell/

Note: I will include CI config in a subsequent PR, this one is from an external fork, so things work slightly differently.