[diffusion] doc: consolidate documentation (#21373)

docs/diffusion/usage.md

@@ -0,0 +1,17 @@
+# Usage
+
+Use this section for day-to-day inference workflows with SGLang Diffusion.
+
+- [CLI](api/cli.md): run one-off jobs with `sglang generate` or start a server with `sglang serve`
+- [OpenAI-Compatible API](api/openai_api.md): request format, endpoints, and SDK examples
+- [Post-Processing](api/post_processing.md): frame interpolation and upscaling
+- [Quantization](quantization.md): quantized transformer checkpoints and supported quantization families
+
+```{toctree}
+:maxdepth: 1
+
+api/cli
+api/openai_api
+api/post_processing
+quantization
+```