DXR Language-Independent Schema

As DXR gains the ability to index more languages, we're going to need a non-C++-centric schema to hold the data. I don't think the existing schema is horrible, but I'm sure there are C++-isms hiding out in there. This page is a place to enumerate those isms and develop alternatives.

Strawman: jcranmer's documentation schema preview

I started work on developing a schema for the purposes of adding Doxygen-like support to DXR. [TODO: add details]