docs: add step-by-step user guides with screenshots

[rprabhat]

Summary

Add comprehensive step-by-step guides for all three user types with screenshots and examples:

New Guides Added

1. Explorer Quick Start (docs/getting-started/explorer-guide.md)

   • Step-by-step for non-technical users
   • Screenshots: Home, Graph, Chat, Report views
   • How to download, install, add folder, chat with documents
   • Offline/private messaging

2. Developer Quick Start (docs/getting-started/developer-guide.md)

   • CLI installation from source
   • Project initialization
   • Schema definition example
   • Query writing example
   • HTTP client code snippets

3. AI Agent Integration (docs/getting-started/agent-guide.md)

   • Architecture diagram
   • Schema examples for memory/context
   • Python and TypeScript code examples
   • MCP integration coming soon note

Technical Changes

1. entitlements.plist - macOS hardened runtime entitlements required for notarization
2. tauri.conf.json - Updated signing config, entitlements path, minimum macOS version
3. explorer-release.yml - Added signing/notarization steps with GitHub secrets support
4. mkdocs.yml - Updated navigation with new guide sections

App Signing

The app currently shows "broken" because it's not signed. This PR adds the infrastructure for signing but requires:

• Apple Developer account ($99/year)
• GitHub secrets for APPLE_CERTIFICATE, APPLE_ID, etc.

For now, users can right-click > Open to bypass Gatekeeper on unsigned apps.

Testing

• Verify docs build at https://sensible-analytics.github.io/SensibleDB/
• Check all three guides render with images