Add a "Storage Provider Guides" section to the FOC docs

## Problem

docs.filecoin.cloud has no documentation for operators who want to *supply* storage to Filecoin Onchain Cloud. There is no guidance on how to become a PDP provider: deploy a node, register with Warm Storage, or withdraw earnings. Today this knowledge lives only in scattered field notes and external docs (docs.filecoin.io).

## Proposal

Add a new top-level "Storage Provider Guides" section, after Developer Guides, covering the provider lifecycle:

1. **Install & Run a PDP Node** - full Lotus + YugabyteDB + Curio stack and Warm Storage registration. One page covering both Calibration and Mainnet.
2. **Withdraw from Filecoin Pay** - withdraw available USDFC and swap to FIL. One page covering both filpay-cli and Foundry.
3. **Advanced** - Nginx reverse proxy (TLS termination) and LXD container setup.

Also add a landing-page card linking to the section.

## Naming

Use "Storage Provider Guides" to match the SDK terminology and the FOC marketplace. Use "PDP node" for the running stack.

## Source material

Based on James (tippyflits) battle-tested field [guide](https://docs.google.com/document/d/1fyQkfdoz-kCfJVc12ty_shdahrzSrjR9u6UJjrz2hk4/edit?tab=t.hp0xr8y6dvro#heading=h.t22ync8zm2hx) for PDP providers.