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
Ricardo Quesada