documentation for level builders: docs index, step-by-step guide, file format reference (wad, geometry, objects, textures, audio, testlevel chapters, engine limits), strat vm and script language with the arena as worked example, tools overview, developer notes; readme rewritten as project front page with screenshots; review notes moved to notes/dev

docs/strats/roller-coaster.md

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+# Example: the roller coaster
+
+> **Coming soon.** A roller-coaster mod is in development. This page will walk through it the way
+> [the arena page](arena.md) does, once it is finished. Until then, its lab notes will appear as
+> `notes/mod-coaster.md` and its code under `src/main/java/com/eliasmueller/llamamod/mods/coaster/`.
+
+<!-- TODO (docs): fill in when mods/coaster is merged: overview + screenshots (docs/img/strats/),
+     files table, the track geometry, the cart/player strat scripts step by step, options, tests,
+     lessons learned. Link notes/mod-coaster.md and the 18-mods.md section. -->
+
+## What is already known about the original rides
+
+* Catacombs chapters 1, 3 and 5 (`t1l7m001`, `t1l7m003`, `t1l7m005`) are roller-coaster rides. In
+  these levels the player is not the `Llama` strat but a **`RollerCoaster`** strat
+  ([Objects](../reference/objects.md#special-objects)).
+* The rides have the longest views of the game: fog end up to 23 cells
+  ([Engine facts](../reference/engine-limits.md#drawing)).
+* The ride music is track 25, `ROLLER` ([Music](../reference/audio.md#music)).
+* Objects of these levels can be imported into other levels, but they only work with matching
+  geometry ([manual chapter 9](../manual/09-objects.md#limits-and-things-to-know)).