Added documentation and updated README

docs/pl_rx_subsystem.md

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+# 📡 PL Rx Subsystem (Channelizer)
+
+## Overview
+
+Implements PFB channelizer and FFT processing.
+
+---
+
+## Architecture
+
+ADC → PFB → FFT → Capture → FIFO → AXI → DMA
+
+---
+
+## Processing
+
+- 4096 MSPS input
+- Decimation 8
+- FFT 512 bins
+
+---
+
+## Output
+
+- AXI4-Stream
+- uint64 format
+- TLAST per frame
+
+---
+
+## 🔗 Related Components
+
+- [PL Tx Subsystem](pl_tx_subsystem.md)
+- [PS Subsystem](ps_subsystem.md)