Separate functions in smaller topics + update ci

[olivroy]

This is to make clearer what operations do what, hence facilitate the selection of the right st_*() function.

This work will require a few iterations to continue, but thought I'd send this PR first to see if there is interest.

The end goal is to combine documentation that applies to many functions in a help topic while separating out the pieces of docs that only apply to a single function to avoid clutter in the doc.

The goal is to have multiple documentation topic and use roxygen2 tags to reuse documentation (like @inheritParams.

Also added a rough classification in the pkgdown index for #2176 (can still be improved)

Fix #2428, seems like this was just a documentation issue!
fix #2176

[olivroy]

@edzer Just a reminder! When you have time, please let me know what you think..

The goal ia to improve the prompts we get when we use autocompletion, to be able to find information more quickly by dividing topics up in more digestible pieces.

A lot of information is present already. The goal of this PR (and the follow-up ones I am planning) is to reorganize slightly by taking advantage of roxygen2 reusability features.

If you want, I can add review comments to help you follow more easily?