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/README_TO_GENERATE_DOCS.md

@@ -0,0 +1,32 @@
+## Prerequisites
+
+To generate the docs for FlatBuffers from the source files, you
+will first need to install two programs.
+
+1. You will need to install `doxygen`. See
+   [Download Doxygen](http://www.stack.nl/~dimitri/doxygen/download.html).
+
+2. You will need to install `doxypypy` to format python comments appropriately.
+   Install it from [here](https://github.com/Feneric/doxypypy).
+
+*Note: You will need both `doxygen` and `doxypypy` to be in your
+[PATH](https://en.wikipedia.org/wiki/PATH_(variable)) environment variable.*
+
+After you have both of those files installed and in your path, you need to
+set up the `py_filter` to invoke `doxypypy` from `doxygen`.
+
+Follow the steps
+[here](https://github.com/Feneric/doxypypy#invoking-doxypypy-from-doxygen).
+
+## Generating Docs
+
+Run the following commands to generate the docs:
+
+`cd flatbuffers/docs/source`
+`doxygen`
+
+The output is placed in `flatbuffers/docs/html`.
+
+*Note: The Go API Reference code must be generated ahead of time. For
+instructions on how to regenerated this file, please read the comments
+in `GoApi.md`.*