[Doc] Update vLLM version in doc

[wangxiyuan]

Correct vLLM version in doc

• vLLM version: v0.12.0
• vLLM main: https://github.com/vllm-project/vllm/commit/v0.12.0

[gemini-code-assist]

Code Review

This pull request updates the vLLM version across several documentation files. The changes are a good step towards updating the repository for the new version. However, I've noticed a few inconsistencies where the version number has not been updated, specifically in the 'Getting Started' sections of the README files and the 'Release window' section in the versioning policy. I've added comments on the release checklist file pointing out these missed updates, as they are now explicitly part of the release process. Addressing these will ensure the documentation is consistent for users.

[gemini-code-assist]

While the 'Branch section' in README.md and README.zh.md has been updated to v0.12.0, the 'Getting Started' section in both files still refers to v0.11.0rc3. To ensure consistency and fulfill this checklist item, please update the 'Getting Started' sections as well.

[gemini-code-assist]

The 'Branch states' section in docs/source/community/versioning_policy.md has been updated. However, the 'Release window' section has not been updated to include the new v0.12.0 release. As per this checklist item, please add an entry for the new version to the 'Release window' table to keep the documentation consistent.

[github-actions]

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