fw gain compression: add docs

docs/en/features_fw/gain_compression.md

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+# Gain compression
+
+<Badge type="tip" text="main (planned for: PX4 v1.17)" />
+
+Automatic gain compression reduces the gains of the angular-rate PID whenever oscillations are detected.
+It monitors the angular-rate controller output through a band-pass filter to identify these oscillations.
+
+This approach is a safe adaptive mechanism for stable aircraft: the PID gains remain unchanged when no oscillations are present, they are never increased beyond their nominal values, and they are bounded by a minimum limit.
+
+Gain compression can help prevent actuator damage and even loss of the vehicle in cases such as airspeed-sensor failure (loss of airspeed scaling) or in-flight changes in dynamics (e.g.: CG shifts, inertia changes), or other failures that could cause the angular-rate loop to become oscillatory.
+
+![Gain compression diagram](../../assets/config/fw/gain_compression_diagram.png)
+
+## Usage
+
+Gain compression is enabled by default ([FW_GC_EN](../advanced_config/parameter_reference.md#FW_GC_EN)).
+
+It should be disabled during fixed wing [manual tuning](../config_fw/pid_tuning_guide_fixedwing.md) to avoid over-tuning.
+It does not need to be disabled when autotuning.
+
+## Parameters
+
+- [FW_GC_EN](../advanced_config/parameter_reference.md#FW_GC_EN)
+- [FW_GC_GAIN_MIN](../advanced_config/parameter_reference.md#FW_GC_GAIN_MIN)

docs/en/features_fw/index.md

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+# Fixedwing-Specific Features
+
+This section lists features that are specific to (or customised for) fixed-wings:
+
+- [Gain Compression](../features_fw/gain_compression.md)