Add sections into function reference page of pkgdown site

[matt-dray]

The function reference page lists all the functions, which can be clicked to see a rendering of the help files.

These can be grouped into sections with short preambles to make it easier for users to understand the intent of the functions. See the example for {dplyr}'s site, for example. These groups can be declared in _pkgdown.yml.

Provide headings for (1) wl_* functions, (2) simulation, (3) 'building block' functions, etc. What should these groupings be?

Makes sense to wait for the function names to be confirmed before doing this.

[chrismainey]

Might not be necessary now given functions are prefixed by type.