Add Doxygen homepage

Here's an example of how to add a Doxygen homepage.

https://github.com/ChrisThrasher/argon/commit/5580ffbb3507adfa99722cd032d5e12ea9af9444 

https://christhrasher.github.io/argon/

This gives us an easy place to write a bunch of welcome docs that users immediately see when opening the documentation website. 