Add dedicated sections for the data model (formula examples), views and
axes (tile mode, records mode, drill-down), the .improv file format
(annotated example), import wizard and headless scripting, and the
command/effect architecture. Link docs/design-notes.md from the "Why"
section. Update build instructions to use `nix build .`.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>