Add Filecoin Pin to the FOC docs: - **Core Concepts → Filecoin Pin**: what it is, how it works, and how it fits the Filecoin Onchain Cloud. - **Getting Started → Quick Start with Filecoin Pin**: adapted from https://docs.filecoin.io/builder-cookbook/filecoin-pin/getting-started.
Add Filecoin Pin to the FOC docs: