feat(docs): version-bound release documentation served at /docs

Adds a docs pipeline so each release has a version-bound Markdown
document (docs/releases/vX.Y.Z.md) rendered publicly in the app:

- sync-release-docs.mjs copies docs/releases/*.md into the toolrate
  public dir and generates index.json before every dev/build
- /docs lists all releases; /docs/:version renders the sanitized
  Markdown (marked + DOMPurify, typography styles)
- template + workflow documented in docs/README.md
- current release (v0.6.0) documented as the first entry

.gitignore

@@ -47,3 +47,6 @@ Thumbs.db
 # Replit
 .cache/
 .local/
+
+# Generated release docs (produced by scripts/src/sync-release-docs.mjs during build)
+/artifacts/toolrate/public/docs/