Added documentation and updated README

docs/pl_tx_subsystem.md

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+# 📡 PL Tx Subsystem (Pulse Generator)
+
+## Overview
+
+The Tx subsystem implements a pulse-based LFM chirp generator using a DDS/NCO architecture.
+
+---
+
+## Architecture
+
+TxPulseStart → pulse_gen_ctrl → tx_active → NCO → Output
+
+---
+
+## Chirp Generation
+
+Δφ[n] = Δφ[n−1] + step  
+φ[n]  = φ[n−1] + Δφ[n]
+
+---
+
+## Parameterization
+
+- Fc (center frequency)
+- B (bandwidth)
+
+Derived:
+f_start = Fc − B/2  
+step    = B / (N − 1)
+
+---
+
+## Pulse Timing
+
+|<------ PRI ------>|
+|<-- pulse -->| idle |
+
+---
+
+## Trigger
+
+Controlled via TxPulseStart (from PS subsystem)
+
+---
+
+## 🔗 Related Components
+
+- [PS Subsystem](ps_subsystem.md)
+- [PL Rx Subsystem](pl_rx_subsystem.md)