Added documentation and updated README

docs/ps_subsystem.md

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+# 🧠 PS Subsystem
+
+## Overview
+
+Handles control and processing.
+
+---
+
+## Control
+
+- Writes parameters
+- Triggers Tx (TxPulseStart)
+
+---
+
+## Processing
+
+DMA → unpack → complex → RMS/peak
+
+---
+
+## Execution
+
+- Event-driven
+- Frame drops possible
+
+---
+
+## 🔗 Related Components
+
+- [PL Tx Subsystem](pl_tx_subsystem.md)
+- [PL Rx Subsystem](pl_rx_subsystem.md)