docs: point installers at 2.1.0

[wshobson]

Summary

• describe 2.1.0 as the published 38-tool release
• point release installers and Cargo installation at 2.1.0
• update the documentation contract assertions

Verification

• cargo fmt --check
• cargo test --test project_docs --locked
• CodeRabbit CLI: no findings
• public release, MCPB, and crates.io package verified before updating URLs

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📥 Commits

Reviewing files that changed from the base of the PR and between eef73fd and 0657f8a.

📒 Files selected for processing (2)

• README.md
• tests/project_docs.rs

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