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/FlatBuffers.md

@@ -1,9 +1,12 @@
-# FlatBuffers
+FlatBuffers    {#flatbuffers_index}
+===========
 
-FlatBuffers is an efficient cross platform serialization library for C++, Java,
-C#, Go, Python and JavaScript (C, PHP & Ruby in progress).
-It was originally created at Google for game development and other
-performance-critical applications.
+# Overview {#flatbuffers_overview}
+
+[FlatBuffers](@ref flatbuffers_overview) is an efficient cross platform
+serialization library for C++, C#, Go, Java, JavaScript, PHP, and Python
+(C and Ruby in progress). It was originally created at Google for game
+development and other performance-critical applications.
 
 It is available as Open Source on [GitHub](http://github.com/google/flatbuffers)
 under the Apache license, v2 (see LICENSE.txt).
@@ -26,7 +29,7 @@ under the Apache license, v2 (see LICENSE.txt).
     projects where spending time and space (many memory allocations) to
     be able to access or construct serialized data is undesirable, such
     as in games or any other performance sensitive applications. See the
-    [benchmarks](md__benchmarks.html) for details.
+    [benchmarks](@ref flatbuffers_benchmarks) for details.
 
 -   **Flexible** - Optional fields means not only do you get great
     forwards and backwards compatibility (increasingly important for
@@ -76,7 +79,7 @@ In this context, it is only a better choice for systems that have very
 little to no information ahead of time about what data needs to be stored.
 
 Read more about the "why" of FlatBuffers in the
-[white paper](md__white_paper.html).
+[white paper](@ref flatbuffers_white_paper).
 
 ### Who uses FlatBuffers?
 -   [Cocos2d-x](http://www.cocos2d-x.org/), the #1 open source mobile game
@@ -118,22 +121,23 @@ sections provide a more in-depth usage guide.
 
 ## In-depth documentation
 
--   How to [build the compiler](md__building.html) and samples on various
-    platforms.
--   How to [use the compiler](md__compiler.html).
--   How to [write a schema](md__schemas.html).
--   How to [use the generated C++ code](md__cpp_usage.html) in your own
-    programs.
--   How to [use the generated Java/C# code](md__java_usage.html) in your own
-    programs.
--   How to [use the generated Go code](md__go_usage.html) in your own
-    programs.
--   [Support matrix](md__support.html) for platforms/languages/features.
--   Some [benchmarks](md__benchmarks.html) showing the advantage of using
+-   How to [build the compiler](@ref flatbuffers_guide_building) and samples on
+    various platforms.
+-   How to [use the compiler](@ref flatbuffers_guide_using_schema_compiler).
+-   How to [write a schema](@ref flatbuffers_guide_writing_schema).
+-   How to [use the generated C++ code](@ref flatbuffers_guide_use_cpp) in your
+    own programs.
+-   How to [use the generated Java/C# code](@ref flatbuffers_guide_use_java_c-sharp)
+    in your own programs.
+-   How to [use the generated Go code](@ref flatbuffers_guide_use_go) in your
+    own programs.
+-   [Support matrix](@ref flatbuffers_support) for platforms/languages/features.
+-   Some [benchmarks](@ref flatbuffers_benchmarks) showing the advantage of
+    using FlatBuffers.
+-   A [white paper](@ref flatbuffers_white_paper) explaining the "why" of
     FlatBuffers.
--   A [white paper](md__white_paper.html) explaining the "why" of FlatBuffers.
--   A description of the [internals](md__internals.html) of FlatBuffers.
--   A formal [grammar](md__grammar.html) of the schema language.
+-   A description of the [internals](@ref flatbuffers_internals) of FlatBuffers.
+-   A formal [grammar](@ref flatbuffers_grammar) of the schema language.
 
 ## Online resources