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

docs/source/WhitePaper.md

@@ -1,4 +1,5 @@
-# FlatBuffers white paper
+FlatBuffers white paper    {#flatbuffers_white_paper}
+=======================
 
 This document tries to shed some light on to the "why" of FlatBuffers, a
 new serialization library.
@@ -124,4 +125,4 @@ offered by .proto files in the following ways:
 -   A parser that can deal with both schemas and data definitions (JSON
     compatible) uniformly.
 
-
+<br>