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/modules/ROOT/pages/plug-in.adoc

@@ -0,0 +1,27 @@
+= Helgobox Plug-In
+
+.What is the Helgobox Plug-In?
+____
+include::partial$plug-in-desc.adoc[]
+It is complemented by the xref:app.adoc[].
+____
+
+== Adding a new plug-in instance
+
+The plug-in is fired up just like any other VST instrument in REAPER: By adding an instance of it to an FX chain:
+
+. Right-click the empty space in REAPER's track control panel
+. Select menu:Insert virtual instrument on new track...[]
+. Double-click entry "VSTi: Helgobox - ReaLearn & Playtime (Helgoboss)"
+
+Initially, you see the xref:plug-in/user-interface.adoc[plug-in window].
+
+== Opening an existing plug-in instance
+
+To open an existing plug-in instance:
+
+1. Scroll to the REAPER track which contains the instance.
+2. Press the btn:[FX] button of that track. This opens the track's FX chain.
+3. Click on the Helgobox FX instance.
+
+You will see the xref:plug-in/user-interface.adoc[plug-in window] again.