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.xzap

@@ -0,0 +1,36 @@
+	.INSERT "S4FREQ"
+
+
+	.INSERT "S4DAT"			; DATA IS IN THIS FILE
+
+
+	.INSERT "MISC"
+
+
+	.INSERT "PARSER"
+
+
+	.INSERT "SYNTAX"
+
+
+	.INSERT "VERBS"
+
+
+	.INSERT "GLOBALS"
+
+
+	.INSERT "EARTH"
+
+
+	.INSERT "VOGON"
+
+
+	.INSERT "UNEARTH"
+
+
+	.INSERT "HEART"
+
+
+	.INSERT "S4STR"
+
+	.END