Documentation rework

[vredchenko]

Documentation migration is complete. Here's a summary of what was done:

Files migrated (21 files):

• tutorials/installation.md → getting-started/for-developers.md
• reference/cli-reference.md → agent/cli-reference.md
• 19 files from how-to/ → appropriate sections (backend/, agent/, operations/, development/)

Index files created/updated:

• getting-started/index.md, backend/index.md, agent/index.md
• operations/index.md, development/index.md, architecture/index.md
• athena/index.md, decision-records/index.md

Root navigation:

• Updated index.md with audience-based cards (Developers, Operators, Integrators)
• Component cards (Backend, Agent, Athena)
• Cross-cutting cards (Operations, Development, Architecture)
• Renamed terms-and-defs.md → glossary.md

Deleted:

• Old index files: explanations.md, how-to.md, reference.md, tutorials.md
• Old directories: tutorials/, how-to/, reference/

The new structure follows ADR-0012's audience/component-based organisation instead of the old Diataxis framework.