docs: add FAQ sections to SnippetsLab, Pieces, and Cacher compare pages

[antonreshetov]

Deepens three thinner compare pages by adding an FAQ section to each (for SEO and AI-answer surfaces), matching the newer hub and persona pages.

• All answers grounded in real, verified facts: SnippetsLab JSON import really brings over folders/tags/descriptions/fragments; massCode has no built-in AI (honest on the Pieces page); no direct Pieces/Cacher importer (migration via Gist export); file-level team sharing via shared Git, not a managed team product.
• massCode VS Code extension referenced by these pages verified as real (Marketplace, AntonReshetov.masscode-assistant).
• Cross-links to the new Mac persona page and storage/library docs.