docs: Add pkgdown function reference page

[jack-davison]

Since #902 {leaflet} has used {pkgdown}

One page that wasn't touched was https://rstudio.github.io/leaflet/reference/index.html, which is a massive list of unsorted functions.

This PR attempts to sort them into an ordered list that makes them easier to parse.

It's likely not perfect, but a step in the right direction! I tried to use the articles as a bit of a guide as to how to link the functions thematically.

PR task list:

• News N/A
• Tests N/A
• Update documentation with devtools::document()

[schloerke]

Thank you! This is great!

[jack-davison]

You're very welcome @schloerke! Strangely it doesn't look like its carried over to https://rstudio.github.io/leaflet/reference/index.html though...

[schloerke]

I'm hoping it was a race condition between two website jobs.

I added a commit to main to trigger another build.

[jack-davison]

Seems like it's failed again - very bizarre as it works locally

[schloerke]

Ah! Found where it was going: https://github.com/rstudio/leaflet/tree/4e438c9025ad83b1797c4fc080c0b7fbeb72a8c9/dev

Turned off dev mode here: 39ffbca

🎉 : https://rstudio.github.io/leaflet/reference/index.html

Thank you again