[Doc] Add versioning_policy doc (#62)

### What this PR does / why we need it?

This patch add the versioning policy doc for vllm-ascend

Reference:
- https://spark.apache.org/versioning-policy.html
- https://docs.openstack.org/project-team-guide/stable-branches.html
- https://github.com/pytorch/pytorch/blob/main/RELEASE.md

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
preview: https://vllm-ascend--62.org.readthedocs.build/en/62/

Signed-off-by: Yikun Jiang <yikunkero@gmail.com>

docs/source/index.md

@@ -50,4 +50,5 @@ features/supported_models
 :caption: Developer Guide
 :maxdepth: 1
 developer_guide/contributing
+developer_guide/versioning_policy
 :::