Vendor helgoboss/helgobox (ReaLearn) as basis for custom UI fork

Stripped upstream git history; starting point for replacing the native
SWELL/Win32 mapping UI with something more suited to bulk editing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

plugin-reaper-realearn/doc/helgobox/antora.yml

@@ -0,0 +1,10 @@
+name: helgobox
+title: Helgobox Reference
+version: ~
+nav:
+  - modules/ROOT/nav.adoc
+start_page: introduction.adoc
+asciidoc:
+  attributes:
+    # For making menu and button macros work
+    experimental: true