Remove legacy V0 usage docs

[DevinVinson]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

• remove the openhands/usage/v0/ documentation pages from the docs site
• add a /openhands/usage/v0-deprecated landing page for users arriving from old bookmarks and links
• update docs.json to remove the V0 navigation section and redirect former V0 routes to the deprecation page
• update remaining references in V1 docs that previously pointed readers to the Legacy (V0) section
• regenerate llms.txt and llms-full.txt so removed V0 docs are no longer included

This PR description was created by an AI agent (OpenHands) on behalf of the user.

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Project	Status	Preview	Updated (UTC)
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