Add further structure to function sections if possible

[athowes]

In PR #456 we moved the function sections around in the pkgdown to present a more understandable picture of the functions a user might engage with.

If there were some way to go further and make higher level section headers to e.g. denote that some functions (family, formula, priors, Stan code) will only need to be engaged with if you're adding a new model then I think this could be good.

Ties into broader conversation about exporting functions #79.