docs: Add initial Pressure Advance tuning document

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/prints/square.scad

@@ -0,0 +1,14 @@
+// Test square
+//
+// Generate STL using OpenSCAD:
+//   openscad square.scad -o square.stl
+
+square_width = 5;
+square_size = 60;
+square_height = 5;
+
+difference() {
+  cube([square_size, square_size, square_height]);
+  translate([square_width, square_width, -1])
+    cube([square_size-2*square_width, square_size-2*square_width, square_height+2]);
+}