docs(advance): add Add a New Speculative Decoding Method guide

[Oxygen56]

Motivation

The PyTorch engine has a clean plug-in surface for speculative decoding (BaseSpecProposer + SPEC_PROPOSERS registry in lmdeploy/pytorch/spec_decode/proposers/base.py), and four shipped methods register against it: eagle, eagle3, deepseek_mtp, qwen3_5_mtp. The user-facing docs/en/advance/spec_decoding.md teaches usage of those four names but never explains how to add a fifth, so users have asked the question externally.

Closes (partially) the docs side of #1738 and #4530.

Modification

Adds docs/en/advance/spec_decoding_new_method.md and a toctree entry for it in docs/en/index.rst, right next to spec_decoding.md.

The page covers:

1. The registry / base class / method string triad.
2. The build_specdecode_proposer entry point and why proposers/__init__.py must import the new class.
3. What BaseSpecProposer already provides so contributors don't re-implement weight loading, draft forward, decoding-input update, or fallbacks.
4. A minimal MyMethod(BaseSpecProposer) skeleton with @SPEC_PROPOSERS.register_module(name='my_method').
5. The 3-tuple return contract for get_outputs (draft token ids, model_metas, target_hidden_states).
6. When to override build_model, illustrated with the two in-tree precedents (Qwen3_5MTP shares the target embeddings; Eagle3 swaps embeddings conditionally and widens get_target_hidden_size).
7. A 5-item shipping checklist.

No code changes. All snippets and references point to symbols that exist in lmdeploy/pytorch/spec_decode/proposers/.

BC-breaking

None — docs only.

[RunningLeon]

It covers the same part as in #4589.