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/Support.md

@@ -1,4 +1,5 @@
-# Platform / Language / Feature support
+Platform / Language / Feature support    {#flatbuffers_support}
+=====================================
 
 FlatBuffers is actively being worked on, which means that certain platform /
 language / feature combinations may not be available yet.
@@ -39,3 +40,5 @@ Primary authors (github)       | gwvo   | gwvo   | ev*/js*| rw     | rw     | ev
   * js = jonsimantov
   * mik = mikkelfj
   * ch = chobie
+
+<br>