Render custom stan functions using Doxygen

[seabbs]

Is your feature request related to a problem? Please describe.
The current stan documentation (added in #422) follows the suggested format in the stan docs which themselves are based on Doxygen. However, this documentation is not currently rendered so is hard to navigate and in some cases read.

Describe the solution you'd like
Using Doxygen to render our documentation similarly to done here. Note there is no official stan support and this example is the only one I have seen. Once this is in place we would also need to clearly link from the main package site.

Describe alternatives you've considered
A different documentation system but it is not clear to me what this would be.

@sbfnk @jamesmbaazam might be interested in this for EpiNow2.