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/playtime/modules/ROOT/pages/introduction.adoc

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+= Introduction
+
+include::helgobox::partial$intro-header.adoc[]
+
+|===
+|Last update of text: |`2025-03-15 (v2.18.0)`
+|Last update of screenshots: |`2025-03-15 (v2.18.0)`
+|===
+
+== What is Playtime?
+
+link:https://www.helgoboss.org/projects/playtime[Playtime] is a powerful and versatile *session view* or *clip launcher* made specifically for the DAW link:https://www.reaper.fm[REAPER]. It is designed as a creative instrument for recording and triggering both audio and MIDI clips, making it ideal for *live performances*, *improvisation*, *jamming*, *looping* and *sketching arrangements*.
+
+As part of the link:https://www.helgoboss.org/projects/helgobox[Helgobox] plug-in,
+Playtime integrates seamlessly with REAPER by leveraging its unique native-speed extension API. This deep integration provides a smoother and more efficient user experience compared to using REAPER alongside other DAWs.
+
+For a more detailed introduction to Playtime and a shiny presentation of its features, visit the link:https://www.helgoboss.org/projects/playtime[Playtime website].
+
+== About this document
+
+This reference provides a detailed guide to Playtime's usage, concepts, and features.
+
+[TIP]
+====
+
+**You also might want to check out the link:https://github.com/helgoboss/helgobox/wiki/Playtime-Home[Playtime Wiki]!**
+
+It is the place for user-contributed content and contains links to other related content, such as tutorials and guides.
+====