[Doc] Add Triton operator description specification template (#14966)

## Why

There is currently no unified template for documenting Triton operators
under `vllm_ascend/ops/triton`. New operator docs tend to be written in
an ad-hoc way, making them hard to review and search.

## What

Add a specification template at
`vllm_ascend/ops/triton/docs/triton_description_specification.md`, which
defines a standardized structure for each Triton operator doc:

- **Description**: function, formula, algorithm flow, supported modes
- **Parameters**: table of parameter name / input-output-attribute /
description / data type / data format
- **Constraints**: shape, dtype, value range, constraints, and
graph-mode support of each input
- **Origin and Differences**: which vllm operator it is derived from,
and the NPU/vllm-ascend specific adaptations
- **Test Cases**: where single-operator accuracy tests should be placed,
and an example pytest command

## Notes

- Naming rule: docs are placed under `vllm_ascend/ops/triton/docs` and
named after the operator; add a suffix if the name already exists.
- Items that do not apply should be marked as "N/A" instead of left
blank.
- Test cases must use the actual shapes/parameters from the inference
scenario rather than arbitrarily constructed ones.

- vLLM version: v0.27.1
- vLLM main:
https://github.com/vllm-project/vllm/commit/ba07e4a48fc951300d97eb506217dd530583dea3

---------

Signed-off-by: ZT-AIA <1028681969@qq.com>