Update documentation for 0.8.7 release and security fixes

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Summary by cubic

Updates the documentation for the 0.8.7 security release, covering the credential leak fixes, secret masking, and new features in the upgrade guide and changelog.

• Bumps the homepage banner to point to the 0.8.7 release notes.
• Adds security hardening notes to the 0.7 → 0.8 upgrade guide: password hashes and API tokens no longer appear in listing responses, and gateway/registrar secrets are masked in API responses and admin forms.
• Adds the 0.8.7 changelog entry covering new features (Redis/Memcached cache backend, RDAP availability checks, per-TLD transfer code option), bug fixes, and dependency changes, linking to the full release notes.

^{Written for commit c638147. Summary will update on new commits.}

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	docs	c638147	Commit Preview URL Branch Preview URL	Sep 01 2026, 08:50 AM

[cubic-dev-ai]

No issues found across 3 files

<sub>Auto-approved: Updates documentation for the 0.8.7 security release: bumps the homepage banner, adds security-hardening notes to the upgrade guide, and adds the changelog entry. Pure docs, no code changes.

Re-trigger cubic</sub>