Move docs/ package mirror into document-navigator plugin

Relocate the ~6k-file docs tree from the repo root into
plugins/document-navigator/docs/ so the plugin is self-contained and
portable. docs/superpowers/ (specs and plans) stays at the repo root.

Moved via cp+rm rather than git mv because directory rename() fails with
EPERM on this WSL /mnt DrvFs mount; git rename-detection pairs the
identical-content files at commit time.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugins/document-navigator/docs/threejs/README.md

@@ -0,0 +1,17 @@
+# `threejs`
+
+Three.js and React Three Fiber integrations.
+
+<!-- begin:auto-docs -->
+## Subcategories
+
+1 subcategory, 1 package.
+
+### `r3f`
+
+React Three Fiber networking and scene components.
+
+| Package | Description |
+|---|---|
+| [`@suaveplan/r3f-networking`](./r3f-networking/README.md) | R3F-tier multiplayer state-synchronisation for React-Three-Fiber scenes. Composes any `@suaveplan/network` `Transport` (canonical: `@suaveplan/network-transpor… |
+<!-- end:auto-docs -->