Revamping the FlatBuffers docs.

Adding an API reference for the supported languages.

General docs cleanup, including a new `tutorial` section that
supports all of the supported languages.

Added samples for each supported language to mirror the new
tutorial page.

Cleaned up all the links by making them `@ref` style links,
instead of referencing the names of the generated `.html` files.

Removed all generated files that were unnecessarily committed.

Also fixed the C# tests (two were failing due to a missing file).

Bug: b/25801305

Tested: Tested all samples on Ubuntu, Mac, and Android. Docs were
generated using doxygen and viewed on Chrome.

Change-Id: I2acaba6e332a15ae2deff5f26a4a25da7bd2c954

samples/monster.fbs

@@ -1,10 +1,10 @@
-// example IDL file
+// Example IDL file for our monster's schema.
 
 namespace MyGame.Sample;
 
 enum Color:byte { Red = 0, Green, Blue = 2 }
 
-union Any { Monster }  // add more elements..
+union Equipment { Weapon } // Optionally add more tables.
 
 struct Vec3 {
   x:float;
@@ -20,6 +20,13 @@ table Monster {
   friendly:bool = false (deprecated);
   inventory:[ubyte];
   color:Color = Blue;
+  weapons:[Weapon];
+  equipped:Equipment;
+}
+
+table Weapon {
+  name:string;
+  damage:short;
 }
 
 root_type Monster;