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/suaveplan-package-structure/.claude-plugin/plugin.json

@@ -0,0 +1,5 @@
+{
+  "name": "suaveplan-package-structure",
+  "version": "1.0.0",
+  "description": "Library package structure patterns and code organization rules for Suaveplan monorepo"
+}