docs: PICS wiki page + WebAPI.batch_call section New /wiki/PICS.md — comparison table for PICS vs. store HTTP vs. Web API, batched name/type lookup recipes, access-token gating, change-tracking via get_changes_since. Cross-linked from Home, SteamClient, and the sidebar. WebAPI.md — new "Batching" section documenting batch_call plus the JSON-array pattern for *Service endpoints. SteamClient.md — cross-linked to the new PICS page from the existing "Product info (PICS)" section.