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/GoApi.md

@@ -0,0 +1,26 @@
+Go API
+======
+
+\addtogroup flatbuffers_go_api
+
+<!-- Note: The `GoApi_generate.txt` code snippet was generated using `godoc` and
+     customized for use with this markdown file. To regenerate the file, use the
+     `godoc` tool (http://godoc.org) with the files in the `flatbuffers/go`
+     folder.
+
+     You may need to ensure that copies of the files exist in the `src/`
+     subfolder at the path set by the `$GOROOT` environment variable. You can
+     either move the files to `$GOROOT/src/flatbuffers` manually, if `$GOROOT`
+     is already set, otherwise you will need to manually set the `$GOROOT`
+     variable to a path and create `src/flatbuffers` subfolders at that path.
+     Then copy the flatbuffers files into `$GOROOT/src/flatbuffers`. (Some
+     versions of `godoc` include a `-path` flag. This could be used instead, if
+     available).
+
+     Once the files exist at the `$GOROOT/src/flatbuffers` location, you can
+     regenerate this doc using the following command:
+     `godoc flatbuffers > GoApi_generated.txt`.
+
+     After the documentation is generated, you will have to manually remove any
+     non-user facing documentation from this file. -->
+\snippet GoApi_generated.txt Go API