docs: Restructure documentation with Diátaxis framework

Reorganize docs into four Diátaxis categories for better discoverability:

## Tutorials (learning-oriented)
- Installation, quickstart, and example projects
- Hands-on guides for getting started

## How-to (task-oriented)
- Memory system configuration
- Agent factory and smart router
- Security, webhooks, HIPAA compliance

## Explanation (understanding-oriented)
- Philosophy and multi-agent concepts
- How Claude learns, adaptive learning
- Integration with LLMs and MemDocs

## Reference (information-oriented)
- CLI guide and cheatsheet
- Complete API reference
- FAQ, troubleshooting, glossary

Also:
- Move internal/development docs to docs/internal/
- Create index pages for each section with card grids
- Update mkdocs.yml navigation structure

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: silversurfer562