fix: properly terminate llama.cpp kv_overrides array with empty key + updated doc (#6672)

* fix: properly terminate kv_overrides array with empty key

The llama model loading function expects KV overrides to be terminated
with an empty key (key[0] == 0). Previously, the kv_overrides vector was
not being properly terminated, causing an assertion failure.

This commit ensures that after parsing all KV override strings, we add a
final terminating entry with an empty key to satisfy the C-style array
termination requirement. This fixes the assertion error and allows the
model to load correctly with custom KV overrides.

Fixes #6643

- Also included a reference to the usage of the `overrides` option in
  the advanced-usage section.

Signed-off-by: blob42 <contact@blob42.xyz>

* doc: document the `overrides` option

---------

Signed-off-by: blob42 <contact@blob42.xyz>

docs/content/docs/advanced/advanced-usage.md

@@ -233,6 +233,15 @@ n_draft: 0
 # Quantization settings for the model, impacting memory and processing speed.
 quantization: ""
 
+# List of KV Overrides for llama.cpp (--override-kv flag)
+# Format: KEY=TYPE:VALUE
+# Example: `qwen3moe.expert_used_count=int:10`
+# Use this to override model configuration values at runtime.
+# Supported types include: int, float, string, bool.
+# Multiple overrides can be specified as a list.
+overrides:
+  - KEY=TYPE:VALUE
+
 # Utilization percentage of GPU memory to allocate for the model. (vLLM)
 gpu_memory_utilization: 0