[Doc][Feature] Add vLLM Ascend development guidelines AGETNS.md (#6797)

### What this PR does / why we need it?
This PR adds a new document, `AGENTS.md`, which provides detailed
development guidelines for contributors to the vLLM Ascend project.
These guidelines cover code style, testing, NPU-specific considerations,
and the contribution process to ensure code quality and consistency.

### Does this PR introduce _any_ user-facing change?
No, this is a documentation-only update for developers.

### How was this patch tested?
This is a documentation change and does not require testing.

- vLLM version: v0.15.0
- vLLM main:
83b47f67b1

Signed-off-by: wangxiyuan <wangxiyuan1007@gmail.com>

.pre-commit-config.yaml

@@ -47,6 +47,7 @@ repos:
   rev: v0.45.0
   hooks:
   - id: markdownlint
+    args: ['--fix']
     exclude: '.*\.inc\.md$|.*report_template\.md$|.*contributors\.md$|.*PULL_REQUEST_TEMPLATE\.md$'
     stages: [manual] # Only run in CI