docs(reflow): rewrite README as Diátaxis-shaped index
Group links by the four Diátaxis modes (tutorials, how-to,
reference, explanation) plus a top-of-page pointer to the
getting-started orientation page. Each link has a one-line hook
so readers can decide where to click without opening each doc.
Contributor docs live in the equalify-reflow repo now (not on
this site). Top-level README points there explicitly rather than
keeping the old reflow/contributing.md on a consumer-facing site.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>