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

java/com/google/flatbuffers/Constants.java

@@ -16,12 +16,19 @@
 
 package com.google.flatbuffers;
 
-// Class that holds shared constants.
+/// @cond FLATBUFFERS_INTERNAL
 
+/**
+ * Class that holds shared constants
+ */
 public class Constants {
     // Java doesn't seem to have these.
+    /** The number of bytes in a `short`. */
     static final int SIZEOF_SHORT = 2;
+    /** The number of bytes in an `int`. */
     static final int SIZEOF_INT = 4;
+    /** The number of bytes in a file identifier. */
     static final int FILE_IDENTIFIER_LENGTH = 4;
 }
 
+/// @endcond