Publish Doxygen documentation online on release

[azrogers]

Similarly to CesiumGS/cesium-unreal#1377, we currently don't publish the Doxygen docs that we build for cesium-native. For the reasons listed in the linked issue, it would be a good move for us to publish the cesium-native docs online as well.

[azrogers]

CC @AndieTursi

[azrogers]

Summarizing here all the requests for API documentation that I could find.

First, on GitHub:

1. Add developer documentation #619 - "may I ask why I can't find a developer manual about Cesium Native?"
2. Create C++ API reference cesium-unreal#689 - "Cesium JS and the ion SDK have a documentation page like this available from the Learn menu. Can we create something like this for the C++ codebase for Cesium-Unreal?"

A few posts on the forum about Unreal documentation:

1. Can you make a documentation like UE4 documentation
2. Where can I find the c++ documentation without having to build it myself?

And many more about Unity documentation:

1. Where can I find the API documentation for the Cesium for Unity package?
2. Is there any relevant API documentation, such as which interfaces to call when I want to perform coordinate conversion
3. Where can I find the documentation of Cesium components and their methods to program from C# scripts?
4. Cesium for Unity documentation somewhere?
5. Is there an api for unity
6. Also where can i get unity c# documentation for cesium
7. I also can’t find the API documentation for the CesiumUnity package so I have to read the source code to figure it out
8. is there any link for Scripting API to help me learn the function of each script

Unity would take a bit more effort since we're not currently generating any documentation for the C# side of things. This would just require setting up Doxygen or DocFX as part of the build process. For Unreal and Native, we mostly just need to adapt the JSDoc template from CesiumJS to work for Doxygen - or grab an existing template and slap our logo on it.

@shehzan10

[azrogers]

Couldn't leave it for over the holiday - added Unity Doxygen support in CesiumGS/cesium-unity#471.

[corybarr]

Several times in the past, I would have found online documentation useful.