chore: apply directory structure from main refactor to stable content

Restructure documentation to follow Diataxis framework:
- overview/concepts -> explanation
- developer_guide -> howto/developer_guides
- user_guide -> howto/user_guides + tutorial
- reference/modules -> modules (flat)
- reference/technical -> technical_reference
- community -> community_and_support
- reference/glossary.rst -> glossary.rst

This preserves all content history from stable while adopting
the improved directory structure from main.

Removed content not in new structure:
- overview/features (feature descriptions)
- overview/products (product pages)
- overview/case_studies (case studies)

Author: pmigueld