Doc: Add Docs about EAGLE speculative decoding (#3144)

Co-authored-by: Chayenne <zhaochenyang@ucla.edu>
Co-authored-by: zhaochenyang20 <zhaochen20@outlook.com>

docs/index.rst

@@ -29,6 +29,7 @@ The core features include:
    backend/native_api.ipynb
    backend/offline_engine_api.ipynb
    backend/structured_outputs.ipynb
+   backend/speculative_decoding.ipynb
    backend/function_calling.ipynb
    backend/server_arguments.md