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

samples/sample_text.cpp

@@ -14,11 +14,10 @@
  * limitations under the License.
  */
 
-#include "flatbuffers/flatbuffers.h"
 #include "flatbuffers/idl.h"
 #include "flatbuffers/util.h"
 
-#include "monster_generated.h"
+#include "monster_generated.h" // Already includes "flatbuffers/flatbuffers.h".
 
 using namespace MyGame::Sample;
 
@@ -52,4 +51,6 @@ int main(int /*argc*/, const char * /*argv*/[]) {
   if (jsongen != jsonfile) {
     printf("%s----------------\n%s", jsongen.c_str(), jsonfile.c_str());
   }
+
+  printf("The FlatBuffer has been parsed from JSON successfully.\n");
 }