Add new Polymarket documentation (2026-02-17)

- Added 31 new documentation pages from llms.txt index
- Updated changelog and 4 existing pages
- New categories: polymarket-learn (FAQ, deposits, get-started, markets, trading)
- New API: get-positions-for-a-market
- New guide: builders examples

docs/polymarket-learn/FAQ/why-do-i-need-crypto.md

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+> ## Documentation Index
+> Fetch the complete documentation index at: https://docs.polymarket.com/llms.txt
+> Use this file to discover all available pages before exploring further.
+
+# Why Crypto?
+
+> Why Polymarket uses crypto and blockchain technology to create the world’s largest Prediction market.
+
+Polymarket operates on Polygon, a proof-of-stake layer two blockchain built on [Ethereum](https://ethereum.org). All transactions are denominated in USDC, a US-dollar pegged stablecoin.
+
+This architecture offers several advantages over traditional prediction markets:
+
+## Why USDC?
+
+### Stable Value
+
+Polymarket denominates trades in USDC, which is pegged 1:1 to the US Dollar. This shields you from the volatility associated with other cryptocurrencies and offers a stable medium for trading.
+
+### Regulated Reserves
+
+USDC operates in adherence to regulatory standards and is backed by reserved assets.
+
+### Transparency
+
+Blockchain technology facilitates transparency, as all transactions are recorded publicly.
+
+### Global Reach
+
+Research has shown that wide availability of prediction markets increases their accuracy. Using decentralized blockchain technology removes the need for a central authority in trading, which fosters fairness and open participation around the globe.