Add index of functions to the TOC of the reference manual
Add index of functions to the TOC of the reference manual

stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/stan,stan-dev/stan,stan-dev/stan,stan-dev/math