Move ZIL source and walkthrough to docs/

Move original ZIL source files (.zil, .xzap, .errors) and
walkthrough.md into docs/ to keep the project root clean.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/s4.zil

@@ -0,0 +1,28 @@
+"S4 for
+		  THE HITCHHIKER'S GUIDE TO THE GALAXY
+	(c) Copyright 1984 Infocom, Inc.  All Rights Reserved."
+
+<PRINC "
+ *** THE HITCHHIKER'S GUIDE TO THE GALAXY: Interactive Science Fiction ***
+
+">
+
+<SNAME "S4">
+
+<SET REDEFINE T>
+
+;<OR <GASSIGNED? INSERT-CRUFTY>
+    <DEFINE INSERT-CRUFTY (STR) <INSERT-FILE .STR T>>>
+
+<INSERT-FILE "MISC" T>
+<INSERT-FILE "HEART" T>
+<INSERT-FILE "PARSER" T>
+<INSERT-FILE "SYNTAX" T>
+<INSERT-FILE "VERBS" T>
+<INSERT-FILE "EARTH" T>
+<INSERT-FILE "VOGON" T>
+<INSERT-FILE "UNEARTH" T>
+<INSERT-FILE "GLOBALS" T>
+
+<PROPDEF SIZE 5>
+<PROPDEF CAPACITY 0>