docs(deployment): add standalone no-elasticsearch deployment guide

[KGerasymiva]

Summary

Add standalone (no-Elasticsearch) deployment documentation for CodeMie, covering deployments where Elasticsearch is unavailable or intentionally omitted.

Changes

• Added standalone deployment guide (docs/admin/deployment/standalone/overview.md) with build script, .env.standalone, compose file, feature-availability table, and troubleshooting
• Updated API configuration docs with RETRIEVAL_BACKEND, ADMIN_LOG_LOOKUP_ENABLED, and healthcheck model_provider tip
• Updated customer feature configuration docs with read-only runtime fields for knowledgeBases, datasources, codeIndexing, metricsAnalytics
• Added Standalone FeatureCard to deployment and admin index pages; new sidebar entry
• Added FAQ entries: how to deploy without Elasticsearch, which features are disabled without it
• Noted ES is optional outside the Kubernetes path in data-layer docs; added availability note to user-guide data-source index

Testing

• Tested locally with npm start
• All pages render correctly
• Images display properly
• Internal links work
• Sidebar navigation works

Quality Checks

• npm run check passes (typecheck + lint + commitlint)
• No MDX compilation errors
• No raw angle brackets (<text> must be `<text>`)
• Sidebar references document IDs (not filenames)
• Images stored locally next to content (not in static/img/)
• Commit messages follow Conventional Commits
• No secrets or credentials in documentation

PR title: docs(deployment): add standalone no-elasticsearch deployment guide