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/Struct.java

@@ -18,8 +18,16 @@ package com.google.flatbuffers;
 
 import java.nio.ByteBuffer;
 
-// All structs in the generated code derive from this class, and add their own accessors.
+/// @cond FLATBUFFERS_INTERNAL
+
+/**
+ * All structs in the generated code derive from this class, and add their own accessors.
+ */
 public class Struct {
+  /** Used to hold the position of the `bb` buffer. */
   protected int bb_pos;
+  /** The underlying ByteBuffer to hold the data of the Struct. */
   protected ByteBuffer bb;
 }
+
+/// @endcond