docs: move the long-form docs into docs/, without the superpowers nesting

ARCHITECTURE, PRODUCT and TODO move out of the repository root, and
docs/superpowers/{specs,plans} flattens to docs/{specs,plans}:
"superpowers" names the tool that wrote those documents, not a kind of
document.

No content changes. This commit is the move, plus the roughly thirty
cross-references the move invalidated.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/fetch-fonts.ts

@@ -3,7 +3,7 @@
  *
  *   bun scripts/fetch-fonts.ts
  *
- * Self-hosted, not linked from a CDN, because PRODUCT.md's position is that the
+ * Self-hosted, not linked from a CDN, because docs/PRODUCT.md's position is that the
  * app works offline and needs no third party: a font request to another origin
  * on every cold load contradicts that, and would leak the fact that the app was
  * opened. The files are COMMITTED so neither the build nor a fresh checkout