Change-Id: Ifb955f3daefedae2cb9930a365c4c7e0641776b4

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

Updated CONTRIBUTING.md with some tips.

Change-Id: Id377621a98804293837fda25ef0758cf988eea37

Change-Id: Ie187065698dfb6ba9d989e9d2c48bdd7cb870e89