Remove registry install claims until publishing resumes

[EricSun0218]

Summary

• replace public registry install commands with GitHub Release assets and source project references
• keep English and Chinese READMEs aligned
• remove the stale registry wording from plugin, getting-started, and Unity package docs

Verification

• ./tools/Test-PublicTree.ps1 (378 paths)
• public Markdown contains no NuGet or dotnet add package references