docs(fern): add 0.0.116 migration banner

[pimlock]

Summary

Add a version-specific banner to the OpenShell 0.0.116 documentation announcing the upcoming 0.1.0 release, warning that 0.0.116 will no longer be supported and that upgrading will require migration, and linking to prerelease resources.

This PR targets release/v0.0.116-docs, a maintenance branch created at the exact v0.0.116 tag commit.

Related Issue

No issue required: this is a localized documentation notice for the already-published 0.0.116 release.

Changes

• Add a left-aligned version announcement under the 0.0.116 Fern version entry
• Keep the notice scoped to the 0.0.116 documentation snapshot
• Link to the 0.1.0 milestone, prerelease documentation, and prerelease installation instructions

Testing

• mise run docs passes with 0 errors and 2 existing warnings
• mise run pre-commit passes
• Unit tests added or updated, not applicable to a configuration-only banner change
• E2E tests added or updated, not applicable to a documentation banner

Checklist

• Follows Conventional Commits
• Commit is signed off for DCO

Merging this PR does not publish documentation. The merged maintenance commit must be supplied as source_ref to the updated Sync Docs Website workflow on main, with channel: latest and release_version: 0.0.116.

[github-actions]

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