docs: refresh InvoiceShelf 3 preview guidance

[gdarko]

Summary

• add clear 3.x alpha warnings and direct production users to the supported 2.x line
• document the :next Docker image instead of silently installing stable :latest
• correct Compose URLs, database host, SQLite path, and manual PHP 8.4 requirements
• replace obsolete .dev and npm instructions with the 3.x ./devenv, Node 24, and pnpm workflow
• correct deployment, mobile-app, and Crowdin wording while preserving 3.x-only guides

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