docs: update documentation for v0.6.10

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Purpose

Keep documentation current with the v0.6.10 release.

Docs added/updated

• docs/content/docs/test-sets/adaptive-testing.mdx: New Adaptive Testing workflow page covering generate/evaluate/suggestions APIs and overwrite behavior.
• docs/content/docs/test-sets/_meta.tsx: Added navigation entry for the new Adaptive Testing page.
• docs/content/contribute/backend/authentication.mdx: Documented NIST-aligned password policy defaults, provider policy exposure, and validation behavior.
• docs/content/contribute/environment-variables.mdx: Added password policy env vars and SESSION_SECRET_KEY for backend configuration reference.
• docs/content/sdk/metrics/conversational.mdx: Added ConversationHistory.format_conversation() usage and output example, including tool-call-only assistant turns.
• docs/content/changelog.mdx: Prepended the v0.6.10 release block at the top without changing existing structure.

Codepaths covered

• apps/backend/src/rhesis/backend/app/routers/adaptive_testing.py
• apps/backend/src/rhesis/backend/app/schemas/adaptive_testing.py
• apps/backend/src/rhesis/backend/app/services/adaptive_testing/responses.py
• apps/backend/src/rhesis/backend/app/services/adaptive_testing/evaluation.py
• apps/backend/src/rhesis/backend/app/services/adaptive_testing/suggestions.py
• apps/backend/src/rhesis/backend/app/services/adaptive_testing/tests.py
• apps/backend/src/rhesis/backend/app/services/adaptive_testing/topics.py
• apps/backend/src/rhesis/backend/app/auth/password_policy.py
• apps/backend/src/rhesis/backend/app/routers/auth.py
• apps/backend/src/rhesis/backend/app/auth/providers/email.py
• sdk/src/rhesis/sdk/entities/test_run.py
• sdk/src/rhesis/sdk/entities/test_result.py
• sdk/src/rhesis/sdk/entities/stats.py
• sdk/src/rhesis/sdk/metrics/conversational/types.py
• apps/frontend/src/utils/api-client/adaptive-testing-client.ts
• apps/frontend/src/utils/api-client/interfaces/adaptive-testing.ts
• apps/frontend/src/app/(protected)/adaptive-testing/[identifier]/components/AdaptiveTestingDetail.tsx
• apps/frontend/src/app/(protected)/adaptive-testing/[identifier]/components/SuggestionsDialog.tsx
• apps/frontend/src/components/auth/AuthForm.tsx
• apps/polyphemus/src/rhesis/polyphemus/routers/services.py
• apps/polyphemus/src/rhesis/polyphemus/schemas/schemas.py

Knowledge gaps addressed

• Missing docs for Adaptive Testing overwrite semantics and the suggestion/evaluation workflow.
• Missing backend contributor docs for current password policy defaults and exposed auth policy fields.
• Missing environment variable reference entries for password policy tuning and session secret separation.
• Missing SDK docs for the public conversation transcript formatter used by conversational judges.

Testing

• Confirmed all code examples match source behavior
• No unescaped curly braces in MDX prose
• New directories include _meta.tsx
• Internal links resolve to existing pages