docs: add 0.1.0 upgrade guide outline

[drew]

Summary

Add a concise, audience-based outline for the OpenShell 0.1.0 upgrade guide. The page separates operator actions from end-user changes and stays intentionally brief while the remaining 0.1.0 work lands.

Related Issue

Related to #3063. This draft establishes the guide structure and breaking-change inventory; the remaining definition of done stays open for final release validation and expansion.

Changes

• Add the 0.1.0 guide with separate operator and end-user sections.
• Summarize the reviewed breaking changes with focused migration examples and links to canonical references.
• Link every migration item to its defining PR and identify contracts that are still open.
• Cover external-resource admission, authenticated compute-driver runtime identity, and durable exec launch admission.
• Update workload-image, watch-stream, endpoint-policy, and interactive-exec guidance to match contracts now on main.
• Add an Upgrade section below Security in the docs navigation.
• Add a 0.1.0 announcement and upgrade-guide link to the Overview page.
• Remove the standalone Release Notes page and point the README status badge to GitHub Releases.
• Mark the guide as a working outline while targeted changes remain in flight.

Testing

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

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable; docs-only change)

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