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

CONTRIBUTING.md

@@ -1,42 +0,0 @@
-Contributing    {#contributing}
-============
-
-Want to contribute? Great! First, read this page (including the small print at
-the end).
-
-# Before you contribute
-Before we can use your code, you must sign the
-[Google Individual Contributor License Agreement](https://developers.google.com/open-source/cla/individual?csw=1)
-(CLA), which you can do online. The CLA is necessary mainly because you own the
-copyright to your changes, even after your contribution becomes part of our
-codebase, so we need your permission to use and distribute your code. We also
-need to be sure of various other things—for instance that you'll tell us if you
-know that your code infringes on other people's patents. You don't have to sign
-the CLA until after you've submitted your code for review and a member has
-approved it, but you must do it before we can put your code into our codebase.
-Before you start working on a larger contribution, you should get in touch with
-us first through the issue tracker with your idea so that we can help out and
-possibly guide you. Coordinating up front makes it much easier to avoid
-frustration later on.
-
-# Code reviews
-All submissions, including submissions by project members, require review. We
-use Github pull requests for this purpose.
-
-Some tips for good pull requests:
-* Use our code
-  [style guide](http://google-styleguide.googlecode.com/svn/trunk/cppguide.html).
-  When in doubt, try to stay true to the existing code of the project.
-* Write a descriptive commit message. What problem are you solving and what
-  are the consequences? Where and what did you test? Some good tips:
-  [here](http://robots.thoughtbot.com/5-useful-tips-for-a-better-commit-message)
-  and [here](https://www.kernel.org/doc/Documentation/SubmittingPatches).
-* If your PR consists of multiple commits which are successive improvements /
-  fixes to your first commit, consider squashing them into a single commit
-  (`git rebase -i`) such that your PR is a single commit on top of the current
-  HEAD. This make reviewing the code so much easier, and our history more
-  readable.
-
-# The small print
-Contributions made by corporations are covered by a different agreement than
-the one above, the Software Grant and Corporate Contributor License Agreement.