release: Rivet 0.8.0

[turinglambdaai]

Summary

• cut the pre-1.0 hardening and managed-updater work as Rivet 0.8.0
• keep the Racket package version valid as 0.8 while publishing three-component release version 0.8.0
• move the accumulated unreleased notes under the 0.8.0 changelog heading
• update the English and Chinese version badges
• advance the pinned downstream install examples in docs/ci.md and docs/ci.zh-CN.md to v0.8.0 together with their documentation contract test, per the guides' own same-revision rule

Validation

• raco test tests/documentation.rkt — 168 tests passed
• valid-version? check for the 0.8 package version (mirrors ci.yml gate)
• git diff --check