docs: update release version to 1.7.1

[chaokunyang]

Summary

• update released dependency and installation examples from 1.7.0 to 1.7.1
• update Rust badges and package tables
• add Dart 1.7.1 changelog entries while keeping development metadata on 1.8.0-dev

Validation

• formatted all changed Markdown files with Prettier
• verified user-facing docs contain no stale 1.7.0 or development-version references
• verified active development metadata is unchanged
• ran git diff --check
• local tests and builds were not run; GitHub Actions owns test validation