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/deposits/moonpay.md

@@ -0,0 +1,13 @@
+> ## 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.
+
+# Deposit Using Your Card
+
+> Use MoonPay to deposit cash using your Visa, Mastercard, or bank account.
+
+**Use MoonPay to deposit cash using your Visa, Mastercard, or bank account.**
+
+Access MoonPay by clicking "Buy USDC" on the [Deposit Page](https://polymarket.com/wallet)
+
+Check out [MoonPay's guide](https://support.moonpay.com/customers/docs/how-to-buy-cryptocurrency-with-moonpay) for further instructions.