fix(cli): add error message for 'fern docs preview' command

[devin-ai-integration]

Description

Refs: Slack request from Catherine

When users run fern docs preview without a subcommand, they now receive a clear error message explaining that this is not a valid command and suggesting they use fern generate --docs --preview instead.

Link to Devin run: https://app.devin.ai/sessions/6143cdc4694e4ed4a7f10b7842a1d805
Requested by: Catherine Deskur (catherine@buildwithfern.com)

Changes Made

• Added a default command handler ($0) to the fern docs preview command that displays an error message when no subcommand is provided
• Error message: 'fern docs preview' is not a valid command. To preview docs, use 'fern generate --docs --preview'.
• Added changelog entry for version 3.31.1

Testing

• Lint checks pass (pnpm run check)
• Manual testing of CLI command completed locally

Tested with FERN_NO_VERSION_REDIRECTION=true to bypass version redirection:

$ FERN_NO_VERSION_REDIRECTION=true node packages/cli/cli/dist/prod/cli.cjs docs preview
'fern docs preview' is not a valid command. To preview docs, use 'fern generate --docs --preview'.

Human Review Checklist

• Verify the error message wording is clear and helpful
• Confirm this behavior (error + fail) is preferred over showing help text

[devin-ai-integration]

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