Add common examples

I want to keep the primary documentation digestible, but there are a lot of common use-cases that are a little tricky to figure out. I'm going to start adding those to the examples directory

examples/multimaterial_key_top.scad

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+include <../includes.scad>
+
+/* use this file to generate multimaterial models for making keycaps with a
+   different material for the key top.
+
+   This would be great to use with flexible filament, to make squishy-topped
+   keys.
+
+   You don't need a multimaterial printer to use these files since it's hard
+   height cutoff. You could print the bottom, leave the prints on the bed, and
+   then print the top in a different material. Be careful though! your start
+   gcode may crash into the prints.
+*/
+
+depth = 1;
+// swap the debug()s to render opposite part
+/* debug() */ difference() { // intersection() {
+  key();
+  top_of_key() {
+    translate([-total_key_width(),-total_key_height(),-$total_depth - depth]) cube([total_key_width()*2, total_key_height()*2, $total_depth]);
+  }
+}
+
+debug() intersection() {
+  key();
+  top_of_key() {
+    translate([-total_key_width(),-total_key_height(),-$total_depth - depth]) cube([total_key_width()*2, total_key_height()*2, $total_depth]);
+  }
+}

examples/multimaterial_legend.scad

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+include <../includes.scad>
+
+/* in this example we'll use some openSCAD to generate files for printing inset
+   legends with a multimaterial printer.
+*/
+
+legends = ["F1", "1", "q", "a", "z", ""];
+for (x = [0:1:4]) {
+  translate_u(0,-x) legend(legends[x]) dcs_row(x, 0) {
+    // swap the debug()s to render opposite part
+    debug() key(true);
+    /* debug() */ dished() {
+      legends($inset_legend_depth);
+    }
+  }
+}

examples/single_extruder_multimaterial_legends.scad

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+include <../includes.scad>
+
+/* Don't have a multimaterial printer but still want cool "doubleshot" legends?
+   with a couple tricks, you can! We just have to print upside down with no
+   dish.
+
+   Here's how to use this file:
+
+   1. modify it as you see fit
+   2. render the legends and the keycaps separately
+   3. run the legends through your 3d printer. make sure they are a single layer
+   4. LEAVE THEM on the bed
+   5. change filaments
+   6. run the keycaps over the legends. MAKE SURE they line up! PrusaSlicer
+      centers models on the bed
+   7. voila!
+   8. use a powder-coated bed for extra points
+*/
+
+legends = ["F1", "1", "q", "a", "z", ""];
+for (x = [0:1:4]) {
+  translate_u(0,-x) dcs_row(x, 0) upside_down() legend(legends[x]) {
+    $dish_type = "disable";
+    $inset_legend_depth = 0.2; // whatever layer height you use
+
+    union() {
+      // swap the debug()s to render opposite part
+      debug() key(true);
+      /* debug() */ dished() {
+        legends($inset_legend_depth);
+      }
+    }
+  }
+}