docs: point installation to alpha.9

[SidUParis]

Summary

• point the Chinese quick-install link at the forthcoming Alpha 9 Release
• keep the immutable Alpha 9 source snapshot from directing users back to Alpha 8
• change documentation only; runtime and package metadata are unchanged

Verification

• python3 -m unittest scripts.tests.test_docs_assets — 8 passed
• git diff --check
• current-tree repository secret scan

Release boundary

The public tag and Release will be created only after this PR is merged, all four protected checks pass on the new exact main commit, and the unique exact-main artifact is downloaded and reverified.

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