docs: make the Rust rationale public-focused

[Fail-Safe]

Summary

• refocus docs/why-rust.md on the public decision, measured evidence, limitations, and user-relevant tradeoffs
• move chart regeneration, evidence lifecycle guidance, operational cautions, and open qualification work into a maintainer document
• update the release-readiness language to reflect the completed v0.20.0 cutover

Impact

The Noema website can link readers to a stable public explanation without exposing an inline maintenance checklist. Maintainers retain the procedures and outstanding qualification work in a dedicated guide.

Validation

• make test
• git diff --check
• verified all local links in both changed Markdown files