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/foundation/etl/README.md

@@ -0,0 +1,13 @@
+# `etl`
+
+Extract-Transform-Load pipeline engine with typed stages.
+
+<!-- begin:auto-docs -->
+## Packages in `etl`
+
+1 package.
+
+| Package | Description |
+|---|---|
+| [`@suaveplan/etl`](./etl/README.md) | Extract-Transform-Load pipeline engine with typed stages, error handling, observable progress, and OTel instrumentation. |
+<!-- end:auto-docs -->