Doc: Add Docs about EAGLE speculative decoding

[jhinpan]

Motivation

Add docs in backend for EAGLE speculative_decoding

Modifications

Mainly add speculative_decoding.ipynb for documentation and benchmark

Checklist

• Format your code according to the Code Formatting with Pre-Commit.
• Add unit tests as outlined in the Running Unit Tests.
• Update documentation / docstrings / example tutorials as needed, according to Writing Documentation.
• Provide throughput / latency benchmark results and accuracy evaluation results as needed, according to Benchmark and Profiling.

[qeternity]

We should probably also make clear that EAGLE does not support radix caching at the moment (which will make it unusable for some people).

[zhaochenyang20]

We should probably also make clear that EAGLE does not support radix caching at the moment (which will make it unusable for some people).

Will do.